Hello Rhys,
the appid parameter accepts an Access Token. You use a client ID and client secret to obtain an Acceess Token. Then you can pass the Access Token in the appid parameter.
The steps are described here:
http://msdn.microsoft.com/en-us/library/hh454950.aspx. You have probably completed step 1, so now you jump right to step 2.
Let us know right here, or over at the
Microsoft Translator Developer forum, how it goes.
Chris Wendt
Microsoft Translator