The Peak Assigner is arranged in columns - one for each dimension of your spectrum. The Axis Code (usually H, N, CO or similar) and peak position are provided at the top of each column. Each column contains two panels: the top one contains the NmrAtoms which have been assigned to that peak dimension and the lower panel contains Alternatives, i.e. NmrAtoms which have a similar chemical shift and might be likely assignment candidates. Double-clicking on an NmrAtom in the lower panel will assign it to the peak and the NmrAtom will move to the upper panel. Double-clicking on an NmrAtom in the upper panel will deassign it from the peak and move it to the lower panel.
You can also create a new NmrAtom or directly type in the details of an existing NmrAtom that doesn't appear in the Alternatives box (perhaps because its chemical shift is outside the tolerance limits for that spectrum) by clicking on the New button or right-clicking in the top panel and selecting New nmrAtom. A small pop-up box will appear at your mouse position and you can enter the NmrAtom's Chain, Sequence Code, Residue Type (optional) and Atom Name. You can use Tab to move easily from one box to another. Press Enter or the Accept button to assign the NmrAtom to the peak. If you want to cancel, simply click anywhere outside the pop-up box and it will disappear.
If you wish to edit an NmrAtom in the Peak Assigner simply select the NmrAtom you wish to edit and press the Edit button or right-click and select Edit NmrAtom X. Enter any changes into the pop-up box and then click the Accept button in the pop-up or press the Enter key on your keyboard.