When positioned correctly, you should not feel any pain or discomfort. If you experience any pain or discomfort after insertion, remove your cup and reinsert it according to the instructions. It may take some getting used to (like a tampon).
The stem can be completely inside your vaginal canal or sticking out a little, and that's okay, but the rest of your cup should be inside your vaginal canal. You can always cut your stem to fit your body shape (when you're not wearing it of course, and be careful not to cut the base of the cup).
To make sure your cup is fully open, you can pinch and/or twist the base. You can also run your finger along the base or very gently grasp the stem.