Depending on your answer will determine what to check;
1st - Look at this site: http://www.mercedesshop.com/diy_oil_change2.htm
and make sure you followed all the instructions.
2nd. If you started the motor while draining the oil, never, I repeat, NEVER run the motor when its out of or low on oil or unrepairable damage can and will occur.
3rd. If you did all in 1 correctly check to see that you tightened the oil filter housing down properly, next check to see if when you took the tube out of the top sider if it was the oil in the tube that ran down the block.