Go to: Administration Panel > Users & Groups > Group Administration (under Groups in left column). Select the little gear icon for the moderator group, which should already be created. Scroll down to the bottom of the page and enter the username under 'Group Members'. Next go back towards the top of the page and select the 'Permissions' tab. Under 'Moderator status' in the table below, select the forums that you like this moderator group to moderate.

You can also make a user a moderator without adding them to a group. Just go to: Administration Panel > Users & Groups > Users (under 'Users' in left column) > Select the username from the list. Once you have selected the username, click on the 'Permissions' tab on the right side near the top. Under 'Moderator status' in the table below, select the forums that you would like this user to moderate