First you have to select the items you want to move. You can click on the checkbox in the treeview on the left or hold down Ctrl (or afterwards Shift for a range) and click on the pictures in the group view on the right.
The selection of the picture currently being shown can be toggled by hitting t.
At first, only groups are visible in the tree view. You can make pictures visible, too: in the menu bar, select Show Pics in TreeView.
Right click the item into or after which the selected items should be moved and select the corresponding option in the popup menu.