I used to get that error message, and my solution, was to always "search" for a module before I install it.

ppm> search Tk
[download]

If that doesn't work, it usually means I've used ppm3 before, which usually messes up my ppm set up, so I check, by typing

ppm>set rep
[download]

to see what kind of repositories I have set up, and it usually tells me I have none, so I issue

ppm>set rep default http://www.ActiveState.com/cgibin/PPM/ppmserver.pl
+ex?urn:/PPMServer
[download]

which is the default PPM package repository after which i

ppm> search Tk
[download]

again , at which point everything is back to normal and I can safely

ppm>install Tk
[download]

which usually ( %95 of the time ) works.