I am no wizz on plug-ins, which one did you find and want to give a go Maria? If I know which one, I might be able to give you more specific help.
Did a search and found this site with easy to follow install plugins instructions:
http://www.techzilo.com/how-to-insta...and-gradients/
In the past I have installed a Layers Effect Script that I won't be with out, it gives me so much better control over creating drop shadows then standard one with GIMP.
Install
layerfx.scm from this page
http://registry.gimp.org/node/186 , ones installed it appears in Script-Fu menu > Layer Effects
Hugz Mo