On the column shift, there are two levers that go from the trans up to the "box" on the column. In that box there are two forks (two tyne forks), one for 2nd and 3rd gear and one for reverse and first gear. There is a "tongue" on the end of the gearshift lever shaft that sits between these two forks. When the shift lever is in neutral, the two forks are lined up and that tongue rests in the fork for 2nd and 3rd. The lever is spring loaded so it keeps it there when the car is neutral. When you go to shift to reverse or 1st, you first pull the lever back and the tongue goes into the rear fork (Reverse and 1st) so when you pull it up it turns the fork, which pulls the rod on the trans into Reverse. Pull it back and down and it tips the rear fork the other way which pushes the rod down and puts the trans in 1st gear. If the transmission is in either 1st gear or reverse when it's installed, the spring on the lever will keep the lever in the neutral position but now the transmission is in first or reverse. Because the rear fork is now tipped, you cannot pull the shift lever back and the forks do not line up. There are two levers on the end of the steering column. Make sure they are both lined up (lift one up or push one down to line them up but they should both be in the 'center' and lined up. When they are lined up, you should then be able to pull the column shift lever back and shift it with the shift lever. See photo -
The two levers are in the lower right corner of the photo (with a red circle around them). The rear lever is up, indicating the car/transmission is in reverse. If you push that lever down, the transmission will be in neutral. Those levers are out of "sync" with the lever on the steering column. If you move those levers so they both line up, the trans is in neutral. (make sure the brake is set or the wheels blocked if you're on a hill. Since you just installed the trans, I'm assuming the car is on a level surface.) Then check the shift lever to see if it is in neutral. If so, you should be good to go. If not and you still can't move the column shift lever back and forth, you will need to take the cover off the "box" between the two levers and see where that tongue is in relation to the levers. The best way to do this is to line the levers up so the trans is in neutral. Then remove the cover from the "box" between them (four screws hold it on) and put the tongue back between the two forks. The forks themselves are spring loaded and if the tongue slipped out from between them (this happened to me once) you need to pry the forks apart a little (a large screwdriver between them works well) and then position the tongue back between them. The tongue should rest in the bottom fork (front-most fork) when both forks are lined up. That should do it. Let us know how you come out.