You just load a Flight Plan that was created with the FS Flight Planner or another program like FS Commander 9.2 that can save in the FS Flight Planner format.

Create and save your FS Flight Plan, then Save and click OK (to return to FSX). On the CAPTAIN’S INSTRUMENT PANEL, just above the Marker Beacon lights, there is a switch that, when you mouse over it with Cockpit ToolTips turned ON, show "Marker Beacon Sensitivity Switch" which is actually used for switching between NAV and GPS so it's really a "Nav 1/GPS Toggle Switch". In csx737_manual2.pdf you can read about it on page 54 under L16. GPS/NAV SWITCH, like this:

L16. GPS/NAV SWITCH

1.GPS/NAV Switch
GPS/NAV switch to select an additional FSX flight
plan tracking autopilot (Nav 1/GPS (toggle) should
be assigned via FSX Menu-Controls-Assignments).

I've never tried adding an FMC to the CS 737-100/200. I love the old steam gauges and the way you fly them. It's more "hands on".

I've never had any problems with navigation with the 737-100/200/200ADV/200C/200F.

Hope this helps.