apply for Hong Kong visa. I am iranian student in Berlin for 3 years.

I am afraid that you need to post your application to HK Immigration Department directly because Chinese embassy in your country doesn't specify that Iranians can apply for HK visa at Chinese embassy. In this case, you should post your application at least four or six weeks ahead of your trip because it takes so long to handle your application. The address is as bellow:
Receipt and Despatch Sub-unit
Hong Kong Immigration Department
2/F, Immigration Tower
7 Gloucester Road
Wan Chai, Hong Kong