You don't need to work with the source code. If it's anything like the PC version there's a file Mupen64.ini file that can be edited via a text editor.
Copy this at the end of the file and it should detect and run it:
Code:
[01BDCC854D0AB798500E7BC31A24D94F]
Good Name=Animal Forest (U) [t1]
Header Code=0290AEB9-67A3B6C1-C45
If you ever need to add an entry for another game (or mine doesn't work), do this:
* Right-click the game in the ROM Browser list and select "ROM Properties".
* If the MD5 Checksum line is empty, press "Calculate".
* For the first line of the entry, copy the MD5 checksum in a set of brackets.
For example:
[01BDCC854D0AB798500E7BC31A24D94F]
* For the second line, write
Good Name= and whatever you'd like it to be named in the browser.
For example:
Good Name=Animal Forest (U) [t1]
* For the third line, write
Header Code= followed by the text in the Ini Code line of the ROM properties page.
For example:
0290AEB9-67A3B6C1-C45
Not sure if it will fix the problem, but that's the method used for getting other hacks to run.