Temporary files listed by lsof don't exist
On some unix variants, lsof shows files that have been deleted but are still open. More precisely, these files have a “link count” of 0, meaning that there is no directory entry (no link, i.e. no name) that leads to them, but the file data still exists. The file data will be deleted when the file is closed.

The ability of lsof to display files with a link count of 0 depends on the platform, and I can't find anything in the documentation regarding the situation on OS X. Try running lsof +L to see the files' link count, or lsof +L1 to list only files with a link count of 0.

I wouldn't be surprised if the flash plugin created the file and immediately deleted in, it's a simple technique for making the data harder to obtain from outside the application. In other words, it's what the plugin author would do precisely to make it hard to do what you're trying to do. If that's what going on, you could counter it with something like libtrash for OSX.