Thanks to Matthew Weeks, a tutorial guiding through the process of designing an experiment with WEXTOR can be found here: http://personal.centenary.edu/~rmweeks/ACS_tech/proj_WEXTOR.html
Ulf-Dietrich Reips set up a tutorial featuring screenshots and detailed descriptions that shows the complete process from logging in to downloading results. Get it here (pdf, 6.9MB): http://wextor.org/wextor_docs/WEXTOR_tutorial.pdf