For those who originally picked up version 0.2.1 for Pidgin Portable, there's no a MANUAL INSTALLATION Option for you.
First, pick up the zipped version of the plugin at http://microblog-purple.googlecode.com/files/pidgin-microblog-0.2.2.zip
Second, unzip & install the files
into X:\PortableApps\PidginPortable\App\Pidgin (replace X with your USB's drive
letter) & allow overwrites, if any.
Third, fire up Pidgin & enjoy!
For those who aren't willing to take the manual route, you can wait for the official portable release later at http://microblog-purple.googlecode.com/
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UPDATE -- 07.03.2009
I just got word that the version that'll install into Pidgin Portable. You can pick up the installer at http://microblog-purple.googlecode.com/issues/attachment?aid=-1426665532...
Enjoy!