but I don't know how to install from a PPM repository (is there a utility for it?)
If you don't already have the PPM utility, you can use PAR-Dist-InstallPPD.
However, if there are any post-install scripts to be run as part of the PPM installation, then PAR::Dist::InstallPPD won't run them - you'll have to download them and run them yourself.
If you download the ppd files, they will contain the name and location of any post-install scripts that need to be run. Look for any 'INSTALL EXEC' entries. (I just took a look at http://www.wxperl.co.uk/repo29/Wx.ppd and there were no such entries for it.)
There's a version of PPM on CPAN that's probably not worth messing with.
Strawberry Perl ships with a PPM utility that's generally quite serviceable, but the "up-to-date" version of PPM is proprietary and ships only with ActivePerl.