Flex :: Cannot Add Method / Function To Class Of Existing Project?

I'm totally new to Adobe Flex Builder 3. I've been assigned a fully functional project but when I want to add the simplest function to any class, I always got the same error :
Call to a possibly undefined method through a reference with static type

For example, in some random class there is a function defined like this :
public function GetID() : String { return m_strID; }
If I try to define a new one like below :
public function GetIDFoo() :String { return m_strID; }

And then I try to call both of them like this:
trace ("This line is ok : " + oPhysicalScreen.GetID());
trace ("This line gives me an error : " + oPhysicalScreen.GetIDFoo());
[Code] .....