FAQ:I downloaded third party clothes. How do I install them?
First figure out where your makehuman files are, see FAQ:Where are my MakeHuman files found (where is my HOME directory)?. Paths listed here are relative to that location.
If you downloaded clothes, you probably got a set of separate files. For example, if you downloaded the "Nose piercing" example files, you got:
These should all be placed in new directory under data/clothes amongst your makehuman files. So navigate to the clothes directory (on windows, for example "MY DOCUMENTS"\MakeHuman\v1\data\clothes) and create a new directory "nosepierce".
Then copy the files there.