In the older versions you used to import the .obj file, select it then File > Import and select the .mdd file.
In the 2.73 version I don't see that option anymore. Does anyone know how to import .mdd files into Blender with the newer version?
Instead of directly importing an
.mdd file you may want to use a mesh cache modifier.
A mesh cache modifier can read
.pc2 files and then replaces the objects mesh data with the files mesh for a given frame. Modifier properties allow adjustments for how the animation is played back.
If the .mdd option for importing isn't available, load the add-on in User Preferences:
File >> User Preferences >> Add-ons >> Import-Export: NewTek MDD format (Enable the add-on)