Whoa there! The Amtsgericht should be issuing your Mahnbescheid, the Landesarbeitsgericht is for appeals only, and the Arbeitsgericht only issues them in specific labor related cases. Is this a Mahnbescheid directed to a company, and is your company registered in Bavaria? Then you have to send the application to Amtsgericht Coburg - Mahnabteilung.
Here is the site for Bavarian applications:
http://www.mahngericht-bayern.de/You can ask for 5 % interest over the base interest rate (3.19% since 1 July 2007) or - if you can prove it - a higher interest rate, e.g. for an overdraft or a credit.
In your specific case you can claim the interest for the late payment as a Nebenforderung, just make sure that you can prove your damages should your case come to court.