make Freeplane default program to open .mm files in U 11.10

Right click on the file and choose Properties from the menu that appears. The Properties dialog opens.
Click on the Open With tab. A list of applications appears.
Select the default application you want for the file type. If the application is not on the list, use the Add button to add the application to the list.

Thank for your answer, Enrico.
Unfortunately, your recommended procedure does not work. The Add button applies only to the programs that are already in the list Open With-Other Applications. Since the application Freeplane (freeplane.sh) is not on that list, it cannot be added to the list Open With-Related Applications, nor can then be selected as default.

The problem is thus: how can Freeplane be included in those lists (Open With-Other Applications/Related Applications)?

Regards

Hmm.. I don't know how to do this, but you can install Freeplane with the Ubuntu Software Center. By this way, your .mm file will opened automatically by this program. So, do this:
1. Open a terminal by hitting CTRL + ALT + T and run this command for start the USC (or Open it from the dash if you use Unity/Gnome-shell or from the panel menu):
software-center

2. In this program, search for Freeplane and install it. Then double click on the .mm file.

OK. I found the solution.
1. I installed Freeplane with the Ubuntu Software Center.
2. I then installed the program File Types Editor (also known as AssoGiate) from the Ubuntu Software Center.
3. I opened this Editor and added a new type:
General tab:
 - category: Multipurpose files
 - Name: Freeplane
 - description: mindmapping
 - default icon: (browse to select your Freeplane icon)
Related Types tab:
    application/x-freeplane
Filenames tab:
   *.mm

That was that. Now, Freeplane appears directly selected as default program to open .mm files (i.e. you can doluble-click .mm files and they open in Freeplane).

the issue described by the OP still applies to 12.04 and andres solution described in #4 still works as expected.
note that the file typed editor already lists an entry called x-freeplane which also lists *.mm as filename pattern but somehow .mm files are associated directly to freeplane after an additional entry is specified.

i installed freeplane through apt but the filetype association did not work correctly as described in #3.

anyway, thanks a lot for the solution!

I also confirm this bug still exists in 12.04. It should not be marked as solved as there's only a painful workaround.

Actually, I was wrong. Freeplane appears in the "Open with" menu when the file is local. I was trying to open a remote file, which is probably not supported by Freeplane.

note that using 'open with...' context menu is not the same as having a default program for *.mm files declared (or being able to do so).
i can't recall if the context-menu entry of freeplane was present from the beginning or it this was fixed though...
glad that there are working aproaches now even without installing additional software.

You're right. I can select Freeplane in the "Open with" menu, but I cannot select it as the default program. It doesn't appear in the list, even after I clicked "Show other applications". So the bug is still there.