I am remodeling my bathroom and want to put 12x12 ceramic tiles on the floor and walls.
The plaster walls had old plastic tiles from the 50's. I removed the plastic tiles and now there is a layer of adhesive with trowel marks. For the most part it is fairly even, however, in some spots the adhesive came off completely. I really don't want to replace the wall, is there something I can use that will stick to the existing adhesive, be a little forgiving of "divits" and have the strength to hold a 12x12 tile? Or is there something I can do to the wall... without removing it?

If it were me, I would coat the walls with Mapei's HPG (as a vapour barrier) and use Mapei's Ultra-Lite as a thinset. It will help to fill the "divots" and is easy to level. I only mention "Mapei" as it is readily available in my area. "Aldo Chemicals" also make many fine tiling products & both of them have very helpful websites.

Ive Ran Into This Before, What Ive Done Is Just Get You A Scraper From You Local Hardware Store Or Lowe's. Scrap Off What You Can, Mainly The High Spots, When You Trowel The New Mortar On The Wall Use A 1/4 Trowel, And Use Thinset, Im Not Familiar With Mapeis Tile Supplies But That Means Nothing It May Work Just As Well.