On Android it takes the name of the file and you are done. On iOS however it is a bit more complex.

There are a few steps you should take:

1) Double check the font files are included in the Target in XCode
2) Make sure they are included in the step "Copy Bundle Resources" (add files, not folders)
3) Include them in the Info.plist of the app
4) Find the name of the Font through FontBook or with some log statements in your AppDelegate

Explained in more detail here: http://codewithchris.com/common-mistakes-with-adding-custom-fonts-to-your-ios-app/

Good luck!