You can smear a little milk on the end of her nipples to get the taste their but what I would do is milk some out and put it into a clean syringe. Feed it to the baby but very slowly and carefully. I put my finger in the mouth so they suckle at it as I squeeze the milk into her mouth so it doesn't go down the wrong pipe and get into her lungs. I'd go at least 15- 20 cc's, once it gets a good taste of the milk, she'll try harder to find that nipple, and this little bit of colostrum will make her much stronger. Theirs times I do this anyway if for any reason, I just don't want to sit in the stall and wait for the baby to figure it out. I find that if you try to push their head down their, they just push back on your hand anyway, this little easy trick works every time for me. Good luck and let us know how she's doing.
Edited to say, just stick your little finger in the corner of her mouth and she'll start the sucking reflex going, it's actually real easy to do, easy to get the milk into her that way. And the reason I do this quite often is because then I know how much colostrum they get into them, you can do it more than once too. Sometimes they can suck at the nipple and not get any or much, this is a for sure thing.
Milk it into a cup and then pour it into the syringe and yes, just relieving the milk in the mares bag, taking the pressure off will sometimes make the nipples a little bigger and easier to find.