You can install Pidgin from the OpenSolaris.org repository, but there’s no package for the Pidgin-Encryption plugin. Once you point it in the right direction, it’s not hard to install the plugin from source.
pfexec pkg install SUNWgcc SUNWxorg-headers SUNWgnome-common-devel
tar -xzvf pidgin-encryption-3.0.tar.gz
cd pidgin-encryption-3.0
./configure --with-nspr-includes=/usr/include/firefox/nspr/ \
--with-nspr-libs=/usr/lib/firefox/ \
--with-nss-includes=/usr/include/firefox/nss/ \
--with-nss-libs=/usr/lib/firefox/ --prefix=/usr
make
pfexec make install
Restart Pidgin, and go to Tools->Plugins, and enable the Pidgin Encryption plugin.