The SynthEdit help file contains a tutorial on the basics (from SynthEdit press <F1> / Tutorials & Reference).
For a great comprehensive resource on using SynthEdit in depth, check out the free ebook "Visual VST/i-Programming - A Comprehensive Guide to Creating VST-FX and Instruments with SynthEdit" - by Peter Schoffhauzer, David Haupt, and H.G. Fortune.
http://www.wizoobooks.com/
For other tutorials, check out the SynthEdit community page:
http://synthedit.com/community/