Where can I get more information about MathLink?

If you are using Mathematica 2.2, then you should have a booklet entitled MathLink Reference Guide, which covers important topics in writing MathLink programs. If you are using Mathematica 3.0, then similar information is available in The Mathematica Book in Chapter 2.12.

A good reference is also available from the Wolfram Information Center.

http://library.wolfram.com/infocenter/TechNotes/174/

This item is a tutorial about MathLink and includes other examples of MathLink programming. It is a good place to start reading.

The last source of good information about MathLink is the MathLink Developer Kit that should have come with your professional version of Mathematica. It contains documentation and examples that you may find useful.