The way I create an angled datum plane is by creating an axis normal to one of the three main datums and constraining it to the other two datums. Then I select the datum plane tool Icon and it (the datum you are creating) will rotate around the axis you just created. You can adjust your pivot axis by adjusting your offset constraints from the original two datums.
The method Debra describes is pretty much the simplest. You can do a similar method using a geometry edge and face to give the angle though it is more robust to use the first method.
If you need the plane at a compound angle to the original datums then a tidy method is to create an offset datum coordinate system to your default coordinate system and use the Orientation tab (WF4) and choose rotations about X and/or Y and/or Z. Once you have the angles you want then pick the CYS you have just created and select the Datum Plane create tool. You will see that it gives a translation along an axis (default X).