Hi there and welcome to ps3savegame.com!
You can upload saves by filling out the form in "Upload Save". This menulink will be visible when you login. The form itself should be pretty self explanatory. But you need to compress the save folder (zip it into one file) on your computer/USB stick before uploading the file through the form. It doesn't matter what you call the zip archive - it will be renamed anyhow.
Let me know if something is still unclear.