I followed the instructions from the stamps client. I am running osc 2.3. The plugins I've found are only for 2.2 and those aren't going to work. Per the instructions, the stamps client appears to have changed.
Stamps also doesn't provide a "plugin" so to speak, it is just a single file that is uploaded to the admin directory. I know that I shouldn't be making assumptions, but with as much as they update their software I would think that they would support osc 2.3.