I just installed a new transmission, but when I shift into gear the truck doesn't move. It is an automatic transmission. My shift linkage is hooked up. Where else could I have gone wrong????? Anyone have any suggestions???

What year and model and what transmission 2 or 4W?

How much fluid have you put in it?
Does it read full on dip stick?
Is transmission pumping fluid out cooler line with line disconnected?
When you bolted up convertor was there and end play between concvertor and flywheel?
On thing that will kill a new installed transmission is breaking pump gear by letting the convertor sliding out and breaking pump gear when bolted up.

Well, I have a '94 4x4 S10. The tranny is a 4L60e. There is fluid pumping. Though I have kept it a bit low since the truck is still on ramps. As far as the torque converter and flywheel, they are in and all buttoned up.

Check and make sure the 4W is not in neutral position.`
Does it lock drive shaft in park engine off?
Fluid pumping out cooler lines right?
Does it try to take reverse or drive or low?
Try another qt of fluid.
Hoew much fluid have you added?