Talk:Assembler/@comment-162.71.108.61-20130115053430/@comment-4583259-20130115181534

It's the little button near the bottom-right that looks kind of like a double-arrow ">>" or "»", except pointing up. This opens the inventory, and the assembler is the part in the upper-right.

Also read Assembler to see how to move blocks around and how to split stacks.