All the following is for Pluto but should be the same for linuxMCE as far as I know.
You add commands through the Device Template. The best way is to search the commands to find one that already exists, see what command group it is in and add the group. Individual commands can then be switched on/off.
Once this is done then for a C++ device (like xine-plugin) you will need to regenerate the code and recompile.
You would be advised to get a release version of the source and make changes to that rather than work with the latest SVN code. Then try and merge your changes into the latest once you are happy with everything. This will ensure all the supporting libraries etc will compile and be what you expect.
To generate the code, get the DCEGen source and build that. Then run DCEGen from the source directory(which is in the same tree as everything else) with the device number for the template.
You should see placeholders in the code for your commands. Make changes as needed and use the Makefile to build.
You will then want to get the changes back into the main source so that you wont have to do this everytime.