it says that parted, a Gnome app is installed but I don't see it under any of the menus.
Type "parted" into a terminal as root; but I do not recommend it as messing with live partitions is generally a very bad idea. Fetch and burn a Gparted LiveCD instead so you can do your work while the system is down. btw, if the "parted" you mention is really Gparted (it is
available from the CentOS repos), then it should be under Applications > System Tools - but I still recommend using the cd instead.
As for adding space to a volume group, that should be possible but it may need to be done from the command line. The relevant commands are:
pvcreate (turn your new partition into a physical volume first)
vgextend(add the newly created pv to the existing volume group).
Consult "man lvm" for details; or search google now that you know what you're after.