Hi Popcorn,
happy to hear that things work out better now!
For initial descent you can use VNAV, but keep a close eye on it and it´s calculations. For the final approach you must use MCP SPD and either LOC/GS (for ILS) or V/S for NON-PREC or RNAV approaches (using LNAV or HDG SEL). VNAV is not certified for approaches in the 737-300...and while it would work on the real one, it does not (yet) in ours.
Cheers, Jan