Visitors of any religion, and indeed those of none, are always welcome to attend Holy Mass and other services at the cathedral.
Sightseeing is best reserved for other times. The cathedral is usually open to visitors who wish to take a look from 10 in the morning until 5 in the afternoon from Mondays to Saturdays and on Sunday afternoon from 1 till 5.
Guided tours are available on request and may be arranged by calling +49 30 20 34 810 (030 20 34 810 from within Germany).
You can also use the contact form provided here.