ok, for anybody experiencing the same problem here's what was the cause and the solution:
I didn't have the course administration link/block inside the course, period. Therefore I was unable to follow any of the tips from all the prior replies how to change enrolment methods inside courses.
Reason for that weird behavior was that the administration block was set to show on front page only. To change that you need to go to the front page where the administration block is showing, click on "turn editing on" then click on the the settings symbol (the little cog next to the word "administration") and choose "configure administration block". There you will see a section "Where this block appears", inside there's "Page context" dropdown. Choose "Display throughout the entire site". Save changes. Turn editing off.
Now, when going to a course, there will be the "course administration" in the navigation, with all the stuff that one needs to handle a course (including enrolment methods under the "user" link as described above)
Great thanks to all the people that tried to help me out here - simply the basic tools were missing