I ran into this problem again on my linux box.
In order to get the spark fun lib to load automagically from the eagle/lbr/ dir you have to rename it to lower case.
unzip SparkFunEagle-7-28-09.zip to you home dir and then
$ sudo mv SparkFun.lbr /opt/eagle-5.6.0/lbr/sparkfun.lbr
$ sudo chown root.root /opt/eagle-5.6.0/lbr/sparkfun.lbr
I am running 5.6 on jaunty and tiger
BT