If you have Mathcad Prime 7, the converter is built-in. If your Prime version is any of 1 through 6, you need mathcad 15 to be installed on the same PC, and licensed. You can use the very same license file for Mathcad 15 as you used for licensing your Prime.
Once Mathcad 15 is installed you should be able to use the converter.
For installation information see here.
I have a student license for Mathcad, does that mean I have to buy a different license to be able to use the converter?
I guess you mean a student license for Mathcad Prime, right? Usually in this forum we call this simply "Prime" and use "Mathcad" or "Real Mathcad" when we are talking about former version (up to Mathcad 15).
A student license should be good enough to be able to use the converter. If you have a Prime license you can download Mathcad 15 and install it on your machine in parallel to your Prime installation. When installing MC15 you can point it to the already existing Prime license file when it asks for a license file. The Prime license is good for MC15, too.
The converter in the current Prime versions need a full installation of MC15 for the converter to work.
MC15 will be available only until the end of this year! After that MC15 will not be available anymore from PTC site and Prime 8 (or Prime 7.1 ?) is supposed to have an integrated converter which does not need a MC15 installation.