Every wonder what to use to grout mini bricks or tiles into place? The best thing I have found is to use wall spackling paste. The kind you use to fill small holes in real world walls. Spackling paste can be found at Hardware Stores and Home Improvement Stores. I usually add a little gray acrylic paint to tone down the white a little.
Here's a project I did using Houseworks Bricks and Spackling Paste