Several routes:
If you're parsing character separated value files, then you can use simple iostream
ing.
Develop an XLL. Download the Excel SDK and go from there. The framework example in that SDK is pretty good.
Use the COM interface. For this you'll need something like Microsoft's ATL. Low level COM, though possible, is difficult.
Use Apache POI and a JNI / JNA layer to it.
(4) has the advantage that excel doesn't need to be installed so can run well server-side, but it will require Java. (3) is a learning curve if you haven't used COM in C++ before. Budget 6 months of mental fog.
In the absence of any more information, I'd plump for (2). The XLL interface is extremely good.