In case someone else happens across this question through a search it appears that there is a missing step depending upon how PEAR was originally installed (I just used the pkg available on Ubuntu). In order to fix this the PEAR DB must be installed/initialized:#pear install DBdownloading DB-1.7.13.tgz …Starting to download DB-1.7.13.tgz (132,246 bytes)…done: 132,246 bytesinstall ok: channel://pear.php.net/DB-1.7.13Just that simple! That should get you over the hump.

Source: Installation: Pear problem – FreePBX / Installation – FreePBX Community Forums