What kind of modems, (make & model), (pci modem, serial or USB)?
Are you running 64bit Mandriva? (type "uname -ra" in a terminal), if you see "x86_64" you are running 64bit. Only Lucent Win Modem with mars chip set can be installed on Linux 64bit for a pci modem.
If you have a LT modem (Lucent Tech.), you have to install kernel source matching current running kernel and compile drivers. If you are running 32/64bit, visit http://linmodems.org/
and download & use scanModem tool to get all required info to get one of those modems working. If they are serial modems, go through MCC "configure my computer/network" to set it up.
EDIT: After seeing prior post, ppp may not be installed from fresh installation on Mandriva, installing kppp or wvdial (dialers) through "install software, 'urpm'" will install ppp.