using struct

[methodos]

I am examining the code of the Filemaker plug-in SDK.

struct FMX_ExternCallStruct;
typedef struct FMX_ExternCallStruct *FMX_ExternCallPtr;
typedef void (*FMX_ExternCallProc)(FMX_ExternCallPtr);

struct FMX_ExternCallStruct
{
	// Several members. Amongst them:
		short extnVersion;
};

To learn something about using structs I tried using this struct, but I don't succeed in doing so.
I tried:

extern FMX_ExternCallStruct InstanceOfThisStruct;
InstanceOfThisStruct.ExtnVersion = 1;//error C2143: syntax error : missing ';' before '.'
//using the typedef'ed pointer:
extern FMX_ExternCallPtr InstanceOfThisStruct;
InstanceOfThisStruct->ExtnVersion = 1;//same error but with '->' 

I was pretty sure I understood how structs work. Turns out I don't. Anyone a suggestion of how I should be able to use this struct and access a member?

[coder777]

you need to include the definition of the struct in order to use it