GloriaJean - it's very easy to organise this. Just tell your hotel 'receptionist' that you want to hire a mule and driver for the next morning at [whatever time is convenient for you], to take you up to the monstery behind the town. Confirm the price with them. They'll have them waiting for you outside your hotel at the appointed hour.
You can also walk up the hill if you like. I've done both, but you obviously enjoy the scenery a lot more if you are on a mule and not watching your footsteps all the time. At the very top you get a lot of hassle from the kids, but then you get that everywhere in Ethiopia.
There is also another monastery or church (can't remember which) along the road out of town that you can visit in the same way.