Pidgin basically is a GNOME software, seems that you installed using Mandriva Live CD, which installs KDE by default. There is a similar messenger named "Kopete" for KDE. To install Gaim (now called Pidgin) you have to install GNOME libraries for Pidgin.
If you want to install Pidgin in KDE, go to install/uninstall software, search for Pidgin, and when you select it you will be prompted to install dependencies, select them all and click apply. (Even if you have GNOME installed, Gaim/Pidgin should be there by default, if it is not, do the same in Gnome).
If you want to install the meta package GNOME, use this command as root:
This should install GNOME including Pidgin/Gaim.
Yes that's a key point about Gnome etc. What you'll find is that if:
a. you need to install something designed for a particular desktop environment (like Gnome); and
b. you only have a basic Mandriva installation;
you'll have a lot of unmet dependencies. I encountered this when I upgraded from Mandriva 2006 to 2007.1 (the 'Spring Edition'). The key message is to master your package management system. That way 95% of software installs will be (mostly) painless. :D Assuming you have plenty of bandwidth or a useful DVD of your distro.
I think - from reading a lot of previous threads - this is something which can put a lot of new users off in the beginning.