"Can somebody tell me how to correct my code to send a pointer instead of the string? "

Yes!

Your original code:

char numstr; // numstr is a single character
unsigned long num = 500; 
sprintf( numstr, "%lu", num ); // numstr here is a single character;

Corrected Code:

char numstr[LENGTH]; // numstr is an array of LENGTH characters 
unsigned long num = 500; 
sprintf( numstr, "%lu", num );// numstr points to the 1st character in numstr[]

ie, as Craig said, make numstr an Array, so that its name used alone is a pointer!

"I'm not very good at dealing with pointers."
Pointers are fundamental to the 'C' language - you need to get a good book on 'C' and swot-up on them!

"Thanks, I guess I'll have to take a look at the Keil compiler."

It's a 'C' language issue, not a compiler issue!