I'm using Chem3D Std, and when I run an energy minimization using MM2 I get an error saying it can't be completed because some of the parameters are missing.
Chem3D Std has limited MM2 capabilities. The Pro and Ultra version of Chem3D can guess at missing parameters. The Pro and Ultra version of Chem3D also allow you to define missing parameters. Upgrade to the latest version of Chem3D Pro or Ultra.