...but are you trying to read a registry value on the WF server, or on the client accessing the web form? I don't believe you will be able to read the registry on the client machine from a web page; that would be a huge security flaw that the browser would block (or not even provide a mechanism for). It would probably require some sort of ActiveX plugin (assuming IE) or an extension for Firefox.
If it is on the WF server, then you may need to grant READ rights to the referenced key to the "Network Service" account (or whatever ID your WorkFlow is running as) to get this to work.
Full disclosure...I'm a WF newbie, so I could be wrong here, and I apologize if so.