Here's how to use the Join button in the Q3PX plugin:

Click on the Join button.

Enter the path of your ET.exe
file. To do this, click Browse.

Scroll to find the folder that contains you ET.exe file. It should be
something like C:\Program
Files\Wolfenstein - Enemy Territory\. If you have multiple ET
installations like me, then double-click to open the folder of the
installation that you want to use.

Select the ET.exe file and
click Open.

You should see the correct path to the ET.exe file as show from above.
If you are on a network that restricts gaming traffic (like a university
LAN) add +set net_port #### to
the Additional command line
parameters, where #### is a 4-digit number that you choose. This will
probably help your ping. Click once on Close web browser before join game
to select the option that tells the plugin to automatically close your
browser when it loads ET. Clicking again on this button will de-select
it. Click OK.

Click Join. The plugin will
automatically load Enemy Territory and connect to the Beginner's Park
server of your choice.

If you want to change any of the options (such as the ET.exe path) that
you set for the Join button, right-click
on the Join button and choose Options. It will bring back the
Options dialog, and you can make changes.
Suggestions or feedback: PM Pintobean or email him at Pintobean801
<at> gmail <dot> com.