Just in case anyone is interested, if you want to use LaTeX on OSX with Scribus but can't or won't - for whatever reason - use the full (2GB!) installation you can use the smaller BasicTeX instead.
First, go to the MacTeX site
http://www.tug.org/mactex/morepackages.html and download the BasicTeX.pkg package - it's around 64MB.
Install using the downloaded package installer - it's very simple with no confusing options (around 188MB installed).
Then go to Scribus and into "Scribus -> Preferences…", then select "External Tools".
Go to the "Render Frames" box and select LaTeX.
Press the "Change Command…" button and put the command
/usr/local/texlive/2012basic/bin/universal-darwin/pdflatex --interaction nonstopmode in the text box.
You should now be able to use render frames.
I don't know why the default command doesn't work (or didn't for me) which is why I thought I'd write this note in case other people were having problems. If this helps just one other person then it's been worth it.
I've only checked this on OSX 10.6.8 with Scribus 1.4.2 but it should work with other versions.