Depending on a Plug-in

ie. Plug-in A depends on plug-in B.

Open plug-in A's plugin.xml file. On the dependencies tab press the add button. Add plug-in B.

Depending on an External Library

ie. Plug-in A depends on external library 'something.jar'.

Make sure you have set up the net.refractions.udig.libs plug-in as a project. See 2 Plugin Setup for details.
Perform the same as Situation 1, where plug-in B = net.refractions.udig.libs

If the required jar is not a part of the libs plug-in, follow the instructions on under net.refractions.udig.libs.