sudo apt-get install desmumeOr if you're a bit more daring you can be a hero and compile from source.
To start off lets install some dependencies you'll be needing to make this happen.
sudo apt-get install libsdl1.2-dev libgtk2.0-dev libglade2-dev libosmesa6-dev libagg-dev intltool libgtkglext1-dev liblua5.1-0-dev libsoudtouch-devNext we'll need to get the source from the project's Sourceforge page. You can download it here.
Once you get the source you'll need to decompress it. You can double click the archive in your file manager and use the Archive Manage to do the job. Or if you like you can use the terminal command
tar zxfv desmume-0.9.9.tar.gzOnce you have the source in hand you can get ready to make the magic happen. Use the following commands to configure, make, and install Desmume (4 total commands, one per line).
cd desmume-0.9.9Thanks to the nice person @ https://gist.github.com/chrisyco/1470676 for the original guide to compiling Desmume.
sudo make install