How to insert combo-box in Excel?

I'll assume you're using version 2007 of Excel or greater.

You'll need to enable the 'Developer' menu option in the main menu.
(You have 'Home', 'Insert', 'Page Layout' etc along the top of the Excel window - if you don't see 'Developer' - probably at the end - then you'll need to enable it.)

To enable the 'Developer' menu click the 'Office Button' (top left) and then click the 'Excel Options' button. In the 'Excel Options' window make sure 'Popular' is selected on the left - and you should then see an option 'Show Developer Tab In Ribbon' - make sure that option is checked.

Now with the Developer tab enabled on the ribbon select it as the current tab and you'll see an 'Insert' button in the ribbon area. Clicking this will give you a drop-down with all the various form controls - including the 'Combo box'. Just drag and drop it onto your worksheet.

Hope that helps.