Backtracking for a moment, it is easier to build the multiple fixture part if you already have the source files open in FeatureMILL. And here’s why.
To put parts in the dialog box, use Add which opens the Setup dialog box. The Setup dialog box has the parts and setups from all the .fm files that FeatureMILL currently has open or that were selected in Parts List. Using Parts List now is a little more work than already having the files open.
Pick the part and setup in the list that you want to place in the multiple part file.
The fixture ID numbers itself, but you can override it if you wish.
Then click Next. Subsequent Fixture IDs number sequentially from the last Fixture ID input, so you can automate and customize fixture numbering at the same time.
Click Next to open the Configuration dialog box.