problem with the keyboard configuration of a macbook

What is the output of:

env; lsb_release -a; uname -a

Thanks

Here it is:

XDG_VTNR=7
LC_PAPER=es_ES.UTF-8
SSH_AGENT_PID=2817
LC_ADDRESS=es_ES.UTF-8
XDG_SESSION_ID=c3
XDG_GREETER_DATA_DIR=/var/lib/lightdm-data/arnau
SELINUX_INIT=YES
LC_MONETARY=es_ES.UTF-8
CLUTTER_IM_MODULE=xim
SESSION=ubuntu
GPG_AGENT_INFO=/run/user/1000/keyring-aWWkFj/gpg:0:1
TERM=xterm
VTE_VERSION=3409
XDG_MENU_PREFIX=gnome-
SHELL=/bin/bash
SSH_AGENT_LAUNCHER=upstart
WINDOWID=46137356
LC_NUMERIC=es_ES.UTF-8
UPSTART_SESSION=unix:abstract=/com/ubuntu/upstart-session/1000/2762
GNOME_KEYRING_CONTROL=/run/user/1000/keyring-aWWkFj
GTK_MODULES=overlay-scrollbar:unity-gtk-module
USER=arnau
LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arj=01;31:*.taz=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.zip=01;31:*.z=01;31:*.Z=01;31:*.dz=01;31:*.gz=01;31:*.lz=01;31:*.xz=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.jpg=01;35:*.jpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.axv=01;35:*.anx=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=00;36:*.au=00;36:*.flac=00;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=00;36:*.wav=00;36:*.axa=00;36:*.oga=00;36:*.spx=00;36:*.xspf=00;36:
LC_TELEPHONE=es_ES.UTF-8
XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session1
XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0
SSH_AUTH_SOCK=/run/user/1000/keyring-aWWkFj/ssh
DEFAULTS_PATH=/usr/share/gconf/ubuntu.default.path
SESSION_MANAGER=local/arnau-MacBook:@/tmp/.ICE-unix/2878,unix/arnau-MacBook:/tmp/.ICE-unix/2878
XDG_CONFIG_DIRS=/etc/xdg/xdg-ubuntu:/usr/share/upstart/xdg:/etc/xdg
DESKTOP_SESSION=ubuntu
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
QT_QPA_PLATFORMTHEME=appmenu-qt5
LC_IDENTIFICATION=es_ES.UTF-8
PWD=/home/arnau
JOB=dbus
XMODIFIERS=@im=ibus
GNOME_KEYRING_PID=2757
LANG=es_ES.UTF-8
GDM_LANG=es
MANDATORY_PATH=/usr/share/gconf/ubuntu.mandatory.path
LC_MEASUREMENT=es_ES.UTF-8
UBUNTU_MENUPROXY=1
COMPIZ_CONFIG_PROFILE=ubuntu
IM_CONFIG_PHASE=1
GDMSESSION=ubuntu
SESSIONTYPE=gnome-session
SHLVL=1
HOME=/home/arnau
XDG_SEAT=seat0
LANGUAGE=es:en
GNOME_DESKTOP_SESSION_ID=this-is-deprecated
LOGNAME=arnau
DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-qp5gnYSi0g
XDG_DATA_DIRS=/usr/share/ubuntu:/usr/share/gnome:/usr/local/share/:/usr/share/
QT4_IM_MODULE=xim
LESSOPEN=| /usr/bin/lesspipe %s
INSTANCE=
TEXTDOMAIN=im-config
XDG_RUNTIME_DIR=/run/user/1000
DISPLAY=:0
XDG_CURRENT_DESKTOP=Unity
GTK_IM_MODULE=ibus
LESSCLOSE=/usr/bin/lesspipe %s %s
LC_TIME=es_ES.UTF-8
TEXTDOMAINDIR=/usr/share/locale/
LC_NAME=es_ES.UTF-8
XAUTHORITY=/home/arnau/.Xauthority
COLORTERM=gnome-terminal
_=/usr/bin/env
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 14.04 LTS
Release: 14.04
Codename: trusty

Tried SHIFT + 2 to make @

If I press SHIFT+2 I get ", not @.

Then you have a UK keyboard set, sounds like you want the US keyboard.

As you can see here:
http://bioinfo.uib.es/~arnau/keyboard2.jpg

my keyboard is a spanish keyboard.

There is a key between the right "apple command" key and the "left arrow" key, (do not know how it is called). Does that do anything?

Have you tried all combinations like alt-shift-2 etc.?

see also http://askubuntu.com/questions/103220/how-do-i-type-an-at-sign-symbol

Yes, I've tried everything but nothing works.

I'd like to assign a keyboard combination to the keys I'm unable to type.

For example, ctrl+2=@,
and so on...

Any ideas how I can do it?

There are some hints in the askubuntu link that I gave, or you can search for "Ubuntu modify keyboard shortcut" or similar strings in your favorite search engine.