So first of all, to clear off some things, a lot of people believe you have to actually beat the whole game in order to unlock the custom content, but that isn't true. Unlike some other games, Melatonin takes use of the Steam Workshop to have you browse levels.

If you already know what this is, you can skip past this section.
If not:
The Steam workshop is a system that a lot of games use to incorporate custom made addons, and it is what Melatonin uses. To access the steam workshop, you can go click on melatonin in your library, and before starting up your game, go into the workshop tab underneath the game's banner. From there, you should already see a bunch of different levels you can choose from, and subcatagories to find what you are looking for.

Once you have clicked on a level you want, just simply hit the subscribe button (this is pretty much just the download button) and it will immediately start the download for that level. Once you have done that step, you can start up Melatonin and proceed to the main menu. While in the main menu go to Extras>Downloaded Content>The level you downloaded.
If your level isn't there, then it could be because the download may not have finished.

I hope this guide helped! If you have questions feel free to comment, and I will answer them asap.