boeing247 - it sounds like you are not putting all the information in the FMC.

When you select ARR/DEPT you will be able to choose the arrival runway and all the runway information for that runway will be programed including the STAR, if you select one and the proper transition.

If you are in LNAV the plane will follow the magenta line and take you through the arrival. Once on the magenta course for the runway you can check the raw data in the HSI to see how the LOC & G/S are doing. If the final is a long one, delay pushing APP until cleared for the approach or until you are a few miles outside the marker or final fix. This should give you a more stable transition from LNAV to the ILS. Remember to be on speed for where you are on the approach. As you fly inbound to the final fix - flaps 15 / 160k, then as you see the G/S come alive - gear down, flaps 20. Glide slope one dot to go, landing flaps and adjust power to keep bug+5k on final.

Remember you have to put correct information into the FMC or the numbers will be  :D

Garbage in, garbage out!

Here, do some reading....http://loft2047.serverloft.com/B757_200/flight_management_navigation/fms_description.htm