You can use the model command to access models ingame.
1.) Make sure which chars the model can replace without messed anims (use HLMV to view the anims)
2.) Give the mdl file an easy name, like ssj2goku.mdl
3.) Go to your ESF dir, then models then player
4.) Create a new folder, named exactly like the model (without mdl) like ssj2goku
5.) Put the mdl file in there
6.) Ingame, type "model ssj2goku" (without "") in the console and you have the model. When you transform you need to use "model ..." again
7.) If it does not work make sure that LOCK MODELS is unchecked under server settings or on the server you're playing at
PS: You can create a "shortcut" to the model. Type
bind x "model ssj2goku"
in the console, use for the "x" a key of your choice. Better don't use spaces in the model name, remember, use easy ones.