Hi Chris,
Reading over this may help:
Writing iRules look in the sub-topic "Creating, managing, and using data groups".
Location:An external data group must reside in either the /config or the /var/class directory. The default location for storing external data groups is the /config directory.
As far as your formatting issue I think you must specify what it is in the External Class. If it's a host it automatically appends 255.255.255.255 in the background (you will only see this if you use iControl to pull the class members) and if it's a network it appends the assigned netmask:
host 6.6.16.197 := "host1",
host 6.1.17.133 := "host2",
network 192.168.1.1/24 := "network1",