Using Phidgets in Flash/Flex requires the use of the Phidget Webservice for Phidget21. This must be running on the computer on which your Phidgets are connected, which is not neccesarily the computer that your Flash/Flex app is running on.
The PhidgetWebService will automatically act as a Policy file server, and the Phidget Flash library will load this policy file. The Policy file allows access from any host to the port that the webservice is running on. If you need to implement a protected connection, you can use the password functionality of the webservice.