At this step I get the comment “There is not enough free space on Macintosh HD to install.” I have a mid-2011 MacBook Air with 136 GB free on a 500 GB drive ( I upgraded from the original 250 GB). What could be the problem?

Thanks Angela - a very useful guide. I used a fine toothed saw to open up the power supply and a drill to scour out the plastic stopper. White duck tape to keep it closed after I glued it back again. Sure beats buying a new one! Time spent - 45 minutes.

Thanks Darie - a great guide, and it kept my adapter out of the landfill. I used a fine toothed saw to open up the power supply and a drill to scour out the plastic stopper. Keep writing!