Open the executable
The initial page should show the systems from {home}/BattleScribe/data
(can be changed in Options -> Development -> Change sytems folder)
If the system you want to use isn't in there, click "Add" and browse to a catalogue or gst, it will then import all the catalogues from the same folder
Click on the system you want to use (It may freeze for a bit while loading it)
Note:
This is only for testing data, not for making rosters/army lists.
For making army lists, its recommended to use the web version, available at https://newrecruit.eu