Wow, I'm a re-entry EE student in Santa Cruz, had no idea this was possible. Thank you good Sir.
Be careful! Note that the post you're replying to was put up in 2013, it's almost six years old. In that time Prime has progress from 3 to 3.1, 4, and 5. with not a whole lot of improvement--it's still not capable of some of the things that the last pre-Prime version had.
Before you spend a lot of time developing new code for Prime review the dialogs on this site about the development progress and speculation about Mathcad's future. As much as it pains me to say it, your efforts might better be put in another software. Smath (for one) is a free open software mathcad look-alike that welcomes additions and improvements. There are others, including some MatLab free clones
Just for my two cents, Mathcad is pretty cheap for commercial software and it fits my company's needs. In many ways, Prime isn't as good as 15, but 5.0 was a significant improvement. We're pretty happy with it. I'm just looking for more help to write custom functions.
VladimirN, I downloaded your user-efi-dev-studio-project, compiled it and it ran perfectly in Prime 5.0. Have you created any more DLLs for Prime? Also, have you used any mixed languages such as Fortran?