Press the "Change class" button near spawn after the round starts.
For those of you that downloaded the 12w38 snapshot, you're going to need the new version of the map. Due to the changes to the command blocks, some tweaking was required to keep the map functioning properly. There has also been a few class tweaks and a new feature added: Randomize teams!
hum....... maybe when the engineer is selected, will tp to a comand block it changes the gamemode from 2 to 0, so it can place sentrys
But then, everyone's still in adventure mode, so they wouldn't be able to disable it. here's SethBling's explanation on why there is no engineer class on the map:
Quote from SethBling »
2. Make the engineer class. Give it [redstone/levers/buttons/dispensers].
There's a pretty simply reason why I don't do this. Currently, the map is completely resettable between playthroughs. No one has to manually break any blocks to reset it, the redstone does everything automatically. If the engineer places a dispenser or redstone or a lever, that will completely break the resetability. It is potentially possible to have built-in map features that only the engineer can interact with. For instance, maybe it could use flint+steel to light glowstone which triggers a BUD. However, that type of thing tends to scar the map, and make it no longer look like the actual TF2 map.
Some people have suggested using spawn eggs. I don't think there are any spawn eggs that will work very well. A skeleton spawn egg would start shooting your team as well as the other. Similarly, there's a lot of friendly fire, and even a tamed wolf would start attacking your own team. Not to mention that there's no way to prevent the engineer from constantly killing himself to place more and more "turrets". I could put a few "teleporters" into the map that can only be enabled or disabled by engineers, but I think that if the entire point of your class was the enable and disable teleporters, it might not be the most exciting class. I want to get engineer in the game, but there just isn't a viable way to do that yet. We'll see.