Note that the campaign scenarios use different proto & techtree files so if you want to use trainable UMC units in campaign, you will have to rename your data files to the appropriate version number. If you want the AI to be able to train UMC units, you will either have to write new AI files (not explained here) or use Method 2 below. It should be used if you want to train the units for yourself and don't mind if the AI can't train them, or if you want to be able to play online with other people who have the same data files (which is explained in the last few steps).
This method will make units trainable noramlly from buildings of your choice.