"The songs on my ADATs have a 30-second lead-in, and I want my Nuendo or CubaseSX project to start at 0:00:00:00 when the ADAT is at 30 seconds. How can I do this?"

You need to program what is known as a time code offset into your sequencer or audio program. The idea is to make your program understand that you want it to (in this case) treat a time code value of 00:00:30:00 as if it were 00:00:00:00, which is exactly what offsets are designed to do. The procedure that follows is specific to Nuendo and CubaseSX, but it's essentially the same in most programs that can do offsets.

In Nuendo or CubaseSX, go to Project - Project Setup - Display Offset. In this case, set the offset at 00:00:30:00. Now, when your ADAT is at 0:00, your project will read negative 00:00:30:00. When your ADAT is at 0:30, then your project will read 00:00:00:00. If you have audio previously recorded in the project and have just changed the offset, it will also start at -:30. To correct this simply highlight all your tracks with CTRL-A, and drag them to 0:00 in the timeline.