I have a variable with a string. That string has the same name as a movieclip intance. However i connot access the movieclip with the string as an instance?

Use this syntax:

pathToObj[objString]

ex:
_root["helloKitty"] is the same as _root.helloKitty
this["football"] is the same as this.football

So, if you had a movieclip, this.clips.happyClip, you might have:

clipName = "happyClip";
mc = this.clips[clipName];