go to the directory where XS++ exists (you will find XS++.exe and XS++log also there) and add the folder files_to_upload.
Paste your custom files folder there in UNCOMPRESSED FORM(ie without zip or rar) and i guess since you wouldve downloaded the pack it would have the correct folder automatically (folder order being \tpa\preset\custom i THINK)
in xs++ select customization and the select own pack and you will find your files there.