We’ve assembled the Omni drive but the wheels don’t lock down. How do we lock them on? Am I missing a part?
I’m not sure without seeing a picture of your setup (which always helps greatly in these situations), but it sounds like you might need a hub between the wheel and your shaft.
If that’s not it, share a little more detail here or email [email protected] and we’ll get you on the path. 
Sent from my desk at AndyMark
I’ll try to get a picture tonight. Do they normally come with a hub? They just seem to slip off.
http://www.andymark.com/Hub-p/am-0096a.htm was this piece supposed to be included? I’ll try to contact them today.
They don’t come with those, when you ordered them the bottom of the page has usage directions for how to drive them. If you are using hex stock for live axles then those you linked should work. If you are usingoing a dead axle then you can put bearings and a hub with a pulley or sprocket to drive them.
Typically hubs are not included, but for a very good reason.
The hole at the center of the omni wheel is the correct size for a 1/2" inner-diameter (ID) bearing. So teams have the option of putting a bearing into the wheel directly and mounting it on to an axle that does not spin (dead axle), or attaching a hub to an axle that spins in its own bearings (live axle).
I definitely recommend contact AM. They will make sure you get what you need.