Need more information to help you, but I can take a guess at what you're doing wrong:
You replaced the "default" folder with whatever folder was inside the archive (zip or rar) filed you downloaded, right? So your file path looks like "./TTMenu/skin/CustomSkinName"? If that's the case, just rename that custom skin folder to "default" and you should be good to go.
For future reference: The DSTT kernel doesn't support multiple skin selection. You need to keep the "./TTMenu/skin/default folder" and replace the files inside it with the files inside your downloaded archive. If you get a message asking if you want to overwrite the files (BgUp.bmp, etc), then you're going in the right direction.
