Hi cfee,
Unless you have created the part symmetrically about the base planes you will need to create a new reference plane at the mid-point of your part model.
You can then mate or align this plane with the assembly reference plane.
Alternatively, you can use a connect relationship between a keypoint in your part model and an assembly reference plane. Keypoints are the implied points such as centre of circle, end of line, mid-point of line etc.
In V15 onwards you can also position parts using co-ordinate axis systems
To get ref planes to display when placing your part you will need to click the 'Construction Display' icon.
It's the one to the left of the box that says 'Relationship 1' and shows a purple surface with 2 small triangles. When you click it you will get an icon list allowing you to toggle on/off the display of all the different construction elements.
Hope this helps.
beachcomber