Thanks for jumping in, Nick. I'd tried to post a further update with more info but I see it didn't come through.
I'm using Joomla 2.5, and the most recent JomSocial. To test whether it is just an issue with the plugins, I'd also tried installing a Tigra JomSocial template. Same error message, which was actually: "Failed to Move File" and in the box below, "Unable to find install package". Given that they are both related to jomSocial, might this suggest that there's a JS installer that's somehow gone missing?
(and btw, thanks for the link to the permissions tutorial - I've been wondering about that!).
Cheryl