Tailshift bushing kit:
1970 Porsche 914 4-cyl Targa - Shift Linkage - Page 2
The factory adjustment procedure is a bit poorly worded. The general idea is that you change the relative orientations of the front shift rod and the rear shift rod, by loosening the coupler in the center tunnel and tweaking its position.
You can put the shifter into 3rd gear and loosen the coupler and move the gear shift lever so that it just about rests on the spring-loaded 1/2 gate but not quite. Tighten it up, and go back into neutral. Check that the lower part of the gear shift lever is vertical when viewed from the side when the shifter is in neutral. If not, loosen the coupler and move the lever fore or aft until it is, then tighten the coupler again.
You will likely need to repeat this process to get both lateral and longitudinal positions correct. Once they seem good, make sure the coupler is tight and then try to get all six gears. If it works sitting still, take it for a test drive. If all is well, button it up. If not, re-adjust. You may have to set it slightly differently than above--getting all the gears is the most important part, after all.
--DD