1. Press the DEP / ARRIVE button on the CDU. You should then have a page called DEP/ARR Index, if that's the case then press LSK 1 L. Or it may go straight to Departures page. I can't renember.
2. Close any discontinuities by going to the legs page and selecting the fix, then upselecting it to the waypoint above it. For example, if PAE is your first waypoint and there is a discontinuity at KSEA. Click on KSEA then click on PAE.
3. If you have no idea what I'm saying.... try going to the PMDG ( www.precisionmanuals.com ) and downloading there free 747 manual. Read the FMC part... it's more or less exactly the same as in the 737.
4. I find FMCs really easy.... basically, fill out everything with boxes around it then enter route.