I mean were do I put the maps I downloaded from the internet, not from OA...
Make sure that you put the entire .pk3 file, and not only the contents within the file to your /baseoa (Windows) or ./openarena/baseoa (Linux) folder.
It might be, especially on Windows, that your .pk3 files doesn't have the .pk3 extentsion and also looks like .zip files, keep that in mind.