Dimensioning

I am a little confused about this. If I click on linear dims I can only pick reference points at wall corners. It will not let me pick the midpoint of the wall. If I choose aligned dims I can pick both the corners and midpoint of the wall. All my snaps are on. Am I missing something?

You will use Aligned Dimensions 99.99999999999% of the time. You're not missing something, but there is one caveat to keep in mind....

Dimensions in Revit wants... no, needs to "anchor" itself to something, or it won't dimension it. A Midpoint is a Snap Point, not a dimensional anchor point, so you cannot dimension to a Midpoint of anything. The "midpoint" you're talking about is the the "centroid" of the Wall Family, which previews as a dashed blue line. This is built into the Family itself.

Parametric environments dimension much differently than AutoCAD, which only needs a reference. Revit truly dimensions objects, with a real relationship between the object and the dimension, which doesn't compare at all to the Dim Association in AutoCAD.

The trick is this: keep using Aligned Dimensions, but if you need a corner, simply hover over a vertex a hit the Tab key, cycling through the different "anchor points" which will highlight in blue. This is a general tip for all objects. Be reminded that dimensioning walls has more robust tools if you pay attention to the Modify Bar under the Ribbon as you are dimensioning. From here you can choose what face of the wall, or entire wall segments with an Options button to vastly increase accuracy and efficiency.

Hope this helps.