Hello Andreas,
due to the compact form factor some Apple laptops are difficult to open and service. The faulty memory may even be soldered onto the motherboard, hence requiring a complete swap of the motherboard....

Here is a good website with detailed guidelines on how to fix Apple products:

http://www.ifixit.com/Browse/MacBook

4/4 Sport 'Sports Red' (12/2009)

Greetings from Switzerland
Wilhelm