Accessing an external database through pgAdmin installed on my local machine?
The short answer is choose Add server from the file menu and enter the details of the server. However there are many reason's why it may not work. I'll itemize the most common problems:


postgresl isn't listening 
postgresl's pg_hba.conf doesn't allow access from outside 
the server's firewall is blocking access
a router is blocking access


If you run into problems, update your question and please include any error messages you get.

BTW When opening up access to postgres from the outside you may want to enforce the use of SSL.