Making doors is very simple but, at the same time, it does seem to catch a lot of new mappers out. So don't feel bad about it.
There are some tutorials
here. They are quite old and for WAD Author rather than DoomBuilder but they still have good information. However, they don't seem to have a basic door tutorial.
Here are some things that are common problems and which might be worth checking.
Are the lines facing the right way? Door activation lines can only be activated from the front, so make sure the front side of the line is facing the player when he tries to activate the door.
Does the door have the correct activation type? Make sure you give the door a "player uses" activation (not sure the exact term that DB uses for that).
Does the door have a speed? Make sure you have filled in the speed argument on the line. 16 is the same as old fashioned Doom doors, 64 is the speed of fast doors in Doom. You can use them or any speed from 1 to 255.
Also, once you get it sorted, make sure that you set the line to be repeatable (if you want it to be) and that it has a wait time before it closes again (else it will stay open).