server dont see client

Are they LTSP clients?
If not, did you run `sudo epoptes-client -c` on them, as described in http://www.epoptes.org/installation ?

On a client, open a gnome-terminal and type the following two commands:
md5sum /etc/epoptes/server.crt
/usr/sbin/epoptes-client
and paste their output here.

Then run this command on the server:
md5sum /etc/epoptes/server.crt

the output server

refaa1@refaa1-desktop:~$ md5sum /etc/epoptes/server.crt
c7d9fa559142731c1b016c92de13fddb /etc/epoptes/server.crt

output form client work

root@refaa2-desktop:~# md5sum /etc/epoptes/server.crt
c7d9fa559142731c1b016c92de13fddb /etc/epoptes/server.crt
root@refaa2-desktop:~# /usr/sbin/epoptes-client
 * Epoptes-client connecting to server:789...
   ...done.

refaa23@refaa23-desktop:~$ sudo epoptes-client -c 192.168.2.68
[sudo] password for refaa23:
depth=0 C = AU, ST = Some-State, O = Internet Widgits Pty Ltd
verify error:num=18:self signed certificate
verify return:1
depth=0 C = AU, ST = Some-State, O = Internet Widgits Pty Ltd
verify return:1
DONE
Successfully fetched certificate from 192.168.2.68:789

but it not work

output from client not work
refaa23@refaa23-desktop:~$ md5sum /etc/epoptes/server.crt
c7d9fa559142731c1b016c92de13fddb /etc/epoptes/server.crt
refaa23@refaa23-desktop:~$ /usr/sbin/epoptes-client
 * Epoptes-client connecting to server:789... 2012/11/14 08:28:29 socat[2513] E getaddrinfo("server", "(null)", {1,0,1,6}, {}): Name or service not known
refaa23@refaa23-desktop:~$

> * Epoptes-client connecting to server:789... 2012/11/14 08:28:29 socat[2513] E getaddrinfo("server", "(null)", {1,0,1,6}, {}): Name or service not known

That means that this client doesn't know the hostname (="server") of the epoptes server.
Either put it in DNS, or in the client /etc/hosts, or modify /etc/default/epoptes-client as described in the http://www.epoptes.org/installation#TOC-Configuration section.

The simpler way to do that is to add
192.168.2.68 server
in the client /etc/hosts.