Custom maps are only supported by paid version 1.6 and higher. please make sure you are up to date with the latest version of pogo3d.
here is a guide to making custom levels. there is a youtube tutorial in the steam guide too https://steamcommunity.com/sharedfiles/filedetails/?id=3181390270
unfortunately, without an account the steam workshop is READ ONLY! you can’t upload your custom maps to steam workshop if you haven’t purchased the game on steam. but you CAN download them using steamCMD (see https://steamworkshopdownloader.io/)
just make sure to put downloaded maps into %localappdata%low\HEDGEWIZARDS\pogo\custom\maps folder just like you do when making your own maps