First, download the plugin's zip file from the page on blender.org. It's under the "attachments" tab.
Open blender, and in the file menu, click "user preferences"
Click "Install from file".
Find where you saved AddBraid.zip
and select it.
You should be able to find the plugin now. Click the checkbox to enable it.
Then click "add braid" from the menu,
The configuraion panel is by default in the left sidebar at the bottom:
Now you've got a braid!