WSU and UW-L's agreement states that WSU pays for a student's ROTC classes if the student maintains full-time status for a semester with WSU and ROTC classes combined.
The first step to enroll in ROTC is to apply online at https://apply.wisconsin.edu. Note these tips:
To track your ROTC application, go to http://www.ulax.edu/admissions/html/trackapplication.htm
After admission, go to www.uwlax.edu, find WINGS under UW-L Quicklinks, and register for the class you are taking. This must be done before the Friday of the first week of the UW-L semester. Since WSU classes begin earlier, you have plenty of time to register. Do not count on your instructor being willing to sign an Add/Drop slip after that date.