I pulled up the carpet (linoleum, tile) and now I have to get rid of the glue so I can stain my concrete. How do I get rid of the glue?

Unfortunately, this is a question for the flooring people. Determine what type of glue was used if you can, then consult a flooring supplier to find out the "antidote" for that glue. I can tell you that muriatic acid will not break down the glue; it will only give you a headache. Once you get the glue up, you should be able to stain the concrete as long as the concrete wasn't previously sealed.