The most common cause for this problem is that the firmware file truly isn't in the directory where the system is looking for it. Check the following locations:
/lib/firmware/
/usr/local/lib/firmware
/usr/lib/hotplug/firmware
If the firmware is present, but still not being loaded, the following steps may help:
Download DAHDI source tarball. http://downloads.asterisk.org/pub/telephony/dahdi-linux/dahdi-linux-current.tar.gz
Edit the file "Makefile"
Locate the line that starts "HOTPLUG_FIRMWARE:=" and set "HOTPLUG_FIRMWARE:=no"
Compile and install DAHDI as normal from here.
When finished, reload DAHDI or reboot the server. If the driver still throws an error after this step, contact Digium Technical Support.