How can I move a tub drain plunger, if the linkage chain is missing?
Typically these aren't chains at all, but a rod in two pieces. See link provided by Getterdun. If you can grab that lower piece, you can pull it up and completely remove the stopper, by removing the overflow cover. 

I've done this on several tubs over the years, and simply used a rubber stopper in the tub when needed. Newer tubs typically don't use this lever and rod, but have a decorative stopper at the drain itself, harking back to the old rubber stopper on a chain idea, but, of course, costing more, and looking somewhat nicer.