I have PSE 11 installed on my MacPro. Can I also install it on my MacAir?
Yes, you are permitted two activations.
If you purchased from Adobe you should still be able to download after logging-in to My Adobe and browsing your purchase history.
If you have the disk version you will need a friend with a plug-in disk drive or if you have a local apple store they may be able to assist you in mounting the disk.
Since presumably your mac pro has a disc drive (unless you have a preview version of the upcoming model), you can just use that to copy the contents of the disd and then network the computers to copy them over to the air. You do need to do a full install on the air. An installed version of PSE can't be copied over, unlike most other programs.