Upgrade from Basic or Premium to a Pro account to turn on search engine indexing. Without Pro, search engines won't find your albums.
Set it per album
Open the album you want to change, click the three dots at the top and select 'settings'. Check 'Search engine indexing'.
Change the default setting
Open your profile page, go to the 'settings' tab and check 'Search engine indexing'. This applies to new albums.
Want your existing albums to be indexed too? Click the link 'apply these settings to all albums'.