To place components automatically component snap is used. You can find two setting in this feature. You can find the first in your config.pro file. The setting is comp_snap_dist_tolerance (0.5 my setting) this sets the snapping tolerance.
You can find the second one in the component placement dialog box. Expand the snap after selecting the preference if it is not expanded. Check the ‘Activite Snapping’ box.
To use it upon component placement the part will need to be in the assembly window while you are constraining it. Select a mate, align either on the part or the assembly then use Ctrl & Alt to move your component. As you move it various possible references are shown. When you are happy with one release Ctrl & Alt then your part will be constrained.