What do I do when a horizontal rod for a popup drain is too short?
EDIT: Niall C. suggested finding a replacement rod that fits. Home depot has a few sizes on-line, but they may have more in the store. I'd try this solution before custom-making one.

Depending on the design of the horizontal rod, you may be able to replace it with a length of steel rod with the same diameter.

From what I can tell of the parts diagram , you may be able to slide some hardware off of the original rod and back on to your custom cut rod of the same diameter. If the original rod is threaded at one end, you could get a tap and die set or see if threaded rod would work.