I found out that I can install Mono on my Synology DS209. It has a 1,2 GHz Arm compatible processor and luckily Mono is included in the IPKG Packet system. As i’m a Windows developer i use putty to connect to the DS209 from my Windows machine. This might be interesting for other Diskstations: http://www.synology-wiki.de/index.php/Welchen_Prozessortyp_besitzt_mein_System%3F.
So i have made the following steps to install mono:
1. Connect with putty as root to the DS209
2. cd /volume1
3. wget http://ipkg.nslu2-linux.org/feeds/optware/cs08q1armel/cross/stable/syno-mvkw-bootstrap_1.2-7_arm.xsh
4. sh syno-mvkw-bootstrap_1.2-7_arm.xsh
5. ipkg update
6. ipkg install mono
7. Enter mono to check it runs.