I bought an Epson WorkForce 645 printer but cannot seem to install it on my MacBook Air. I tried to download the driver but it didn't take.

Did you use the driver software that came with the printer, or Apple's driver?

As counterintuitive as it may seem, Epson actually reccommends that users of that printer use Apple's drivers rather than their own, on computers running Snow Leopard and Lion.

Edit to my last post...

It may help to read this document from Epson's website, which modifies my earlier response. See first if you have already followed these instructions.

I just finished installing an Epson Workforce 545 on Lion. This info is valid as of early March 2012. The trick seems to be this: DON'T install using the CD that comes with the printer. DON'T download the driver from Epson.

First do an Apple Software Update to make sure the machine is up-to-date. Then simply open the Printer & Scan preferences and add the Epson printer. It will download the driver directly from Apple and you will be good to go. If you do install the bad drivers from Epson, you have to remove the /Library/Printers/EPSON folder and also anything EPSON in the Cache folder, then start over.

Thanks aeropap17. I'd struggled for two hours before I read your post. Installation of my workforce 545 went just fine on my wife's non-Lion MacBook Pro. But my Air with Lion 10.7.3 was having problems. I'd installed from the Epson download. But when I went to add the printer in System Preferences, it would try to download the software from Apple and fail. I followed your instructions and deleted the EPSON folder from /Library/Printers. I wasn't sure how to clear the cache so I restarted in safety mode by holding down the shift key while restarting. Then I added the printer in System Preferences, and this time the driver downloaded and installed without incident. Thanks for the VERY HELPFUL post!!!

Sorry I wasn't clear about clearing the Cache. I believe I was referring to /Library/Caches. Just remove anything that looks like *Epson* from that directory.