To download custom albums in the Words app, go to the Library and tap "Download More At myOsmo" under "Installed" (the top section of the Library). You'll be redirected to the myWords page of the myOsmo website.
Select any of the albums and tap "Add to Words game". You'll be prompted to open the album in the Words game.
Tap the album to include it in the game. Please don't forget to deselect the albums that you don't want to be included. You may need to scroll down and open all of the subsections in your Library to make sure only the right albums are selected.
Then go back to the home screen, tap "Play", and choose any mode to play the album(s) that you have selected.
Please note that custom albums only work in Zen, Zen Junior and Versus modes, and not in the Adventure Mode.
When you're playing an album, you can tap the gear icon in the top left corner to change the settings (which include the ability to hear the words, turn hints on and off, enable lowercase letter recognition, and set difficulty level so that only easier or harder words from the album are shown in the game).
Find more information about custom albums in our PDF guide to the Words app here: