[plt-scheme] problem loading modules

Hi, I'm pretty new to PLT, so maybe this is a dumb question: I'm following
the "Systems Programming with PLT Scheme" tutorial, and when I run it with
(require net/url xml), DrScheme tells me "require: unknown module:
net/url".  This happens with other modules too.  According to the REPL, my
current-load-relative-directory is home, so from there it's (require
Scheme/plt/collects/net/url), which still doesn't work.  And same with
starting from collects/, or plt/, or the absolute path from /home...  So,
does anyone know how to get Scheme to see modules? (If it helps, I'm using
v372 on Ubuntu with mred as the language.  I did the install option where
it's all in one directory.)  Thanks!

Liron Greenstein
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