Mpd implements dynamic IP addresses pool management. When user is authenticated and auth backend have not provided IP address, it can be taken from pool. To make it work you should create one or more address pools with 'set ippool add ...' command and define which pool should be used with 'set ipcp ranges ... ippool ...' command, radius-auth Framed-Pool attribute or ext-auth FRAMED_POOL attribute.
set ippool add pool first last
This command creates new IP address pool if it not exists and adds specified address range to it.