Just ignore the message if you need that set as it doesn't stop anything working.
You just have to click it off everytime.
REALLY??
You have two choices for now
1. Remove the test registry key and use Firefox as before with the unskinned file open dialogs. This is the supported WB setup.
2. Keep the registry key and just ignore that message and use the test dlls.
The registry key is purely there as a short term workaround for Firefox blocking the standard dll names. Due to this it has the side effect of that prompt.