Welcome to the forum!
I can't help with that. But that extension's author participates here, and I'm sure she'll be able to help.
Um.....but my first thought is that this extension was probably made with Inkscape version 0.49 or maybe 0.91. I wonder if you using 0.48 might be part of the problem.
Do you have some particular reason not to upgrade? The current stable version is 0.91, and 0.92 should be coming out, I'm guessing within a month or 2. (Yes, 0.50 through 0.90 were skipped
) If there's no particular reason, you could try upgrading to 0.91 as an easy way to find out if that will fix it.
If not, Moini should be along before too long