Download Jeck Jims model pack, unzip, move the folders in models from models in jeck jims models, to models in Srb2 folder, they should look like this
Sonic Robo Blast 2/models/models folders.
MODSONIC blend, md3, and png should be in this order:
Sonic Robo Blast 2/models/VDLC/PLAY+
then, open the Jeck Jims models.dat and copy line
MODERNSONIC VDLC/PLAY+/MODSONIC.md3 3.0 0.0
These lines VDLC and PLAY+ tell the
game where the model stuff is. They are interchangeable,
but if you move the model stuff, make sure to change
the models.dat location.
paste this into your models.dat if you have your own. If you don't, Just follow the instructions on Jeck Jim's page if you want the whole pack.
If it's just the modernsonic model in Jeck Jims not working, check your models dat and the model's location, or delete the model and download it again, also make sure any unfinished lines.... (writing lines that don't have the character for the model eg:
VDLC/PLAY+/MODSONIC.md3 3.0 0.0 (note how it doesn't specify which character the model is for)) ..... make sure any unfinished lines are at the very bottom with no completed models.dat lines atop of it. Hopefully this helps you, I too struggled with models at the beginning. If this doesn't work, or none of the models work, try turning on opengl as your renderer in settings,/video options/render, set renderer from software to opengl. If that doesn't work, your device might not have opengl support.