The oil cooling fan is attached to the back of the generator so you ALWAYS want the belt properly adjusted. I teach people that if you get a gen light DON'T run the car until you know why, as by the time you lose enough oil pressure to heat to light the other lamp, the motor is gone. I'm assuming this is a factory spec 64 (6 volt). If not or the process below doesn't help, let me know and we'll go from there. Make sure the static battery voltage is at least 6VDC, then start the car and check for charging voltage betweeen 7 and 8.5VDC (won't cut in until you reach 1000 RPM or so). Assuming the battery is good, if you don't have proper charge voltage it's either wires, generator (gen), or voltage regulator (vr). Check ALL the connections to the batt, vr, gen, starter, headlight switch, and dimmer switch as clean and tight. If that doesn't resolve the issue, polarizing the gen will tell you if the likely culprit is the gen or vr. This is simply wiring it up to run as a motor. Take off the belt and disconnect/remove the vr. Run a jumper wire from the DF terminal to the gen case. Grab a set of jumper cables and (doing nothing with the negative side for now) connect the positive battery directly to the D+ terminal on the gen. The gen should spin. If it does, your gen is fine. If it doesn't, disconnect D+ (sorry, just being thorough) for the moment, and now connect the negative jumper cable from negative battery to the gen case and try connecting D+ to battery positive again. If the gen spins now, your gen is fine and the ground strap between the chassis and transmission is bad. Further troubleshooting is beyond two-sentance commentary. it's fairly simple to do, so I would suggest the Bentley Service manual at http://www.bentleypublishers.com/volkswagen/repair-information/vw-type-1-61-65-factory-repair-manual.html (Best $100 you will ever spend on your car), and/or getting someone involved who can actually place their hands on the car.

Sources: wiring diagrams available at thesamba.com

Check your external voltage regulator. Replace belt or adjust the tension on it