There is a quick tutorial that can be found here:
Amplification of multiple modes in Er-doped multimode fibers
Basically you will just need to set the refractive index profile and the ion doping profile. For the simple first case you could just use the built in step index profile and uniform ion density distribution. For the 6 mode case, you will have to generate a file for both the refractive index and ion profile.
In the component properties box for the Er Doped Multimode Fiber, click on Help. There is an explanation of the file format there.
Cheers