When it comes to positioning items on a slide, you can always visit the Transform group on the Properties panel and control such object attributes as the location of a slide object, its size, and rotation.
However, if you prefer to control an object's slide placement without using the Transform panel, Adobe Captivate comes with a few other handy features that are often overlooked: Grids and Guides.
If you select the View menu you'll see four options of note: Show Grid, Snap to Grid, Snap to Object, and Show Drawing/Smart Guides.
If you select Show Grid, little dots will appear on every slide. The dots, which are visual reference points for you to use on the slide, will not display in a published lesson. Using the grid, you can ensure objects are a specific distance from the top, bottom, left, or right of a slide. If you select Snap to Grid, an object will automatically snap to the little dots as you drag the object around the slide.
By choosing Show Drawing/Smart Guides, you will see Smart Guides (green dashed lines) when you reposition, resize, or rotate an object. By using Smart Guides, which are enabled by default, you will be able to align objects with other nearby objects.
Note: Smart Guides do not appear when you try to reposition or resize objects using keyboard nudge controls. If you don't see the Smart Guides as you drag objects around the slide, try zooming closer to the slide (I've found that this often makes it easier to see and use the Smart Guides).
If you would like to see a demonstration of Grids and Smart Guides, check out this video on the IconLogic YouTube channel.