I purchased, and installed on my Mac and Windows - then went to itch.io and I added to my collection, BUT it is asking to pay again. How can I just get it to show up in itch since I already paid for it?

Hopefully you've already figured it out, but anyway you should be able to click the drop-down by your username in the upper-right corner and click "My Library", from here you should find "Things you own" and in that section Asset Forge (Deluxe), just click the download button and you should be redirected to the place to download the most recent version.