In a machine of that size you will likely find a ring at each end of the rotor specifically designed for the addition of trial and correction weights. The ring can typically be seen and will visually present a t-slot with multiple locations at which a weight is inserted, rotated to the desired position, then locked in place with a threaded fastener.
The weights are also usually specific to the make/model of the machine so you might want to check you spare parts or spare parts list in case you have to order them.
Be forewarned that if you search these forums you might find comments from "Becar" that this is a near impossible task. Making the assumption that you understand and have the skills to do multiple plane balancing you should find it a relatively easy task, should it truly be a balance issue.
If you don't have the skills and desire to learn about them check a resource like http://oaktrust.library.tamu.e...ce=1&isAllowed=y. Again, should you lack the skills you might also consider hiring a firm to do the balancing and learn from the balancing effort.