Excel 2013 trouble clicking into toolbar

It (graying-out some of the buttons) is standard behaviour when you are in cell editing mode.

As opposite to post text, your screenshot shows part of your ribbon right from the Font group (where B(old) is placed), so we cant see if is it grayed-out or not. This particular one shall never been grayed out because you can use it for formatting whole cell, as well as single characters in a cell.

Make sure the cursor is cross or cross-arrows shaped. If you have blinking beam in the active cell, you are in cell editing mode and for most commands from the ribbon, you would need to couble click - first to exit cell editing mode, second to select choosen button.