an additive, like Bondex or Weld-Crete, or even a portland cement slurry so that the two will bond tightly. Sometimes, if the concrete is rough or else scarified (chipped with a hammer) it will take the mason-coat. But usually over time it will separate without the above having been applied.
To get a smooth finish: Unless you're skilled at trowelling, it will take you some practice to attain a smooth finish. However, you can put it on heavy, then you and your spouse can take a darby board (length of 2x4 or 1x4) that is straight and starting from the top, pull the board down taking off the excess. However Cyd, you need to perform that task ONLY after the plaster-coat has begun to set. Now when I say "set" I mean after it has firmed enough to scrape on, but is not yet set-up.
As you use the darby, you will discover dipped-in areas that need to be filled-in. Do this with your trowel; let it reset, then darby again. Eventually you will have the wall planed off flat. It takes time. After the final darby has achieved a flat surface, and the mortar has again started to set enough that you can't dent inward with your fingertip, make up a very wet mix (paint consistency) and roll it on with a paint roller. This will fill in any small holes. As that begins to set, but not quite as hard as previously, take your clean trowel and carefully trowel the wall from bottom to top. As you do it, you will notice that the wall undercoat is firmj and it will be like smoothing out butter on toast. Continue until the wall is uniformly smooth.
Good luck with your project. If you need additional help, just e-mail. Hope this helps. ---Jack