If you declare Assault Move into the smoke with just a MMC, it uses 2 MF. To place a DC into a building costs 2 MF. That would be 4 MF which is not allowed if you wish to Assault Move. Since you DID declare Assault Move, you can’t spend more than 1 more MF (and you cant leave your hex). As Fred points out…if you were a SMC or a MMC in a stack with a SMC, you would have 6 MF this turn. So, you could AM into the smoke, place the DC and dance a jig (costs 1 MF) for 4 MF (ok, the jig is 0 MF).