I'd initialize the operand at the top, instead of just declaring it:
string operand = "+";

That way you'd know if it was ever assigned something else. But now that i think about it you'd prolly get an exception if it isn't initialized when you switch on it.

Yeah, use the debugger, but some breakpoint here and there inside btnequals_Click, to see if it getting called at all. The event might have been detached from the button somehow?