i was sent models in mdl format and want to edit them and convert them to other formats what can i do this on in osx

Eh, grammar...

Anyway, you are looking for a modelling program. If you use google, you will find a lot of them, including a few free ones.

Try Blender. It isn't easy to use, but it's free and there are plugins for some common game formats, including .mdl. Google to find the plugins.

Alternatively, just download the latest version of Rotrix and you can play with it on a radial game! I changed my Gargoyle to a bunny on a pogo stick.