Set your Pages v5.6.1 Preferences (Pages menu : Preferences) to the following Ruler settings, and in Pages View menu : Show Rulers.

Once you have done this, you will have horizontal and vertical rulers. As you move your normal document pointer (insertion beam) onto your ruler, you will see it change shape to a pointer. When you click that pointer in the ruler, it will change to a resizing shape, and then you can drag your alignment guide out from your ruler.
Once you have the alignment guide on your page, you can change its orientation from vertical to horizontal, or the reverse, by pressing the option key while double-clicking the alignment guide. You can remove the guides by dragging them back onto their respective ruler, or all guides from the Pages View menu : Guides sub-menu. Also, with a selected object, do not overlook the X, Y coordinates fine alignment capability found on the Format panel : Arrange tab.
More help with Alignment Guides can be found from your Pages Help menu : Pages Help : Add images, shapes, and media : Position objects on a page : Use alignment guides.