If you are using ActivePerl and ppm isn't working, you probably have a broken install. I would first suggest reinstalling ActivePerl. (You may possibly have some other distribution of Perl earlier in your path. Try running

perl -v
[download]

at the command line and see if it mentions ActiveState.)

You could look at doing a CPAN install, but you are going to need a C compiler on your system to do so. You Unicode::String isn't a pure perl module, so you can't (easily) just do a manual install and expect it to work. Your best bet is to get ppm working.