3d coordinates from 2D click?

Hi everybody,

first of all, I am new here and to OpenGl Es, as well, so please bear with me if this has come up before or I am completely wrong forum-wise.

I am used to code with OpenGL but with OpenGL ES I run against walls every few steps. Now I am stuck at trying to create a 3d vector from a 2d click. As in I am coding a small 3d game app and as is to be expected I need the 3d coordinates from a click or touch.
Only that the usual possibilities seem to be not accessible. Like gluUnproject or GL_DEPTH_COMPONENT. So can anybody point me to a solution?

What I am actually trying to do is assessing if and where a click on the screen touches an arbitrarily rotated plane. The plane is not actually rendered to the screen but part of a 3d game object and a helper to determine where a click has occured in relation to the object.

I am thankful for every help, even if it is just a pointer to the right forum :).