When I test an incomplete level (In the making, obviously), any Things which are added don't show up, with errors I can't understand at the top left, namely "PSpawnMapThing; Unknown type 79 at (32, 192)".
I don't get what this is telling me. Could someone clarify?
You're getting that error because there is no Thing Type 79, it used to be the Red Spring but became invalid when the thing numbers switched for 2.0. So this is probably caused by an outdated config.
You can download the correct config file here if you're using SRB2DB or here if you're using Workbench.