Fixed on IRC, but for the record: if you just upgraded from 4.0.14 to 4.1.8 (for example), software lists format changed and PlayOnLinux 4.1.8 find itself with lists in the 4.0.14 format; The main symptom is that "testing" scripts do not show up.
The way to fix that is simple: click on the Refresh button to force lists to be overwritten with 4.1.8 format, and the install window should start behaving correctly.
This also "heals" itself the first time lists need to be updated because they changed on the server.