How do I add pressure and adjust rebound on older mountain bike forks?
You can easilly find the manual just by googling the visible words from your photo "Marzocchi Bomber 2001", for example https://www.manualslib.com/manual/1728712/Marzocchi-Bomber-2001.html  Adjustments are on page 10 and in detail from page 24. A schematic picture is on page 14. 

There are air and coil variants.
What you can adjust in the air ones is the air pressure and the oil level, which in turn determines the air volume.

However from the picture on page 32 it appears to me you have the coil variant and that by getting out the dust cap on the left crown (on the right in the photo) you will get to the coil preload adjustment (use the Allen key). A flat screwdriver should then suffice to adjust the rebound cartridge.

Make sure which exact variant you have. The picture tells a lot.