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:
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.
from this page http://registry.gimp.org/node/186
, ones installed it appears in Script-Fu menu > Layer Effects