You can pick peaks by positioning the mouse where you would like to have one and then right-click the mouse and select Add Peak from the Peak menu. Alternatively, a better way is to drag the mouse over a region while holding down Shift and Crtl – this will place peaks at any maximum in that region. It is worth being aware of the Peak Find Parameters pop-up in the Crosspeaks menu. Here you can define a number of parameters which relate to peak picking. By default only positive peaks are picked, but here you can change this to negative only or positive and negative. Other parameters (which I hardly ever change, but you might find useful) include whether peaks are picked on, above or below the contour level, the picking of diagonal peaks and the method by which the volume of a peak is calculated.
The volume of a peak is calculated at the time when it is picked which means that you do not need to perform an additional peak integration step if you want to get a peak volumes.