u got ur /etc/apt/sources.list entries not uncommented(remove the hash "#" before the lines with universe,multiverse,security etc except deb-src) for universe and multiverse repo may be.do look at
*ubuntuguide.org for details on /etc/apt/sources.list
OR
some problemo with repository may be.u can download the package from ubuntu repo as follows:
*package.ubuntu.com/packagename
as u got feisty(assumes):download below package and install manually(it will be a problem if another depndncy pops up
)
install using gdebi(double click install) or use terminal "cd" into the dir where u downloaded below package and install as "sudo dpkg -i pckgname.deb" format
*mirrors.kernel.org/ubuntu/pool/universe/s/silc-toolkit/libsilc-1.0-2_0.9.12-6_i386.deb
^after installing above package,try installing pidgin again.
then make sure no package is broken by in terminal : "sudo apt-get install -f"
terminal can be opened by right clicking on empty desktop>open terminal or via menu Applications>accsries>terminal.I hope u know basics of terminal usage.