You are correct, there does not appear to be a standalone NDS emulator in the software center.

Also, for future reference, package requests are usually done in this format:

• Name: DeSmuME
• Homepage: http://desmume.org/
• Open Source: Yes
• License: GPLv2
• Link to Source Tarball\Zip file: https://sourceforge.net/projects/desmume/files/desmume/0.9.11/desmume-0.9.11.tar.gz/download
• Description: DeSmuME is a Nintendo DS and Nintendo DS Lite handheld game console (NDS) Emulator.

All the best. ^.^

Sorry, I didn't know that :/ I'll do it in the future :)

No worries, I wasn't sure how to do it either for my first couple requests :P