HELP! Howto install a .gem or a tarball of a gem?

Aloha All,

I have downloaded many gems in .gem and .tar format from Rubyforge to
my dir /usr/local/src/gems/. However, I got the following error below
when executing “gem install mongrel-1.1.5.gem” or any other gems from
the dir above.

“ERROR: could not find gem mongrel-1.1.5.gem locally or in a

I’ve notice that within the .gem folder there are two files
(data.tar.gz and metadata.gz). Hope that helps.

Here is my system layout:
Mac OS 10.5.5 client stock
Dir: /usr/local/
Installed: Ruby (1.8.7-p72) and Rubygems (1.3.1) to /usr/local/

Please help or point me in the right direction on HOWTO install a
download gem from a local directory.

Much mahalo in advance. -Tdub