If I'm not mistaken, the majority of the clothing mods should be a swf file. So, if you wanted to load randomPants.swf, just go to modding tab, click on Swf mod, and find that swf file (in this example, I would look for randomPants.swf)
And yeah, that's how you load clothing mods.
Uhh, I should add a quick note. The way this game saves, it will NOT load swf files the next time you open and click on that character. Instead, you have to create a character folder.
This tutorial created by huitznahua will tell you how to save, but if you have other questions or are confused by the save, I can try my best to help? Idk
SDT Newbie Guide