If reinforcement stacks are in the battle area you should see a little button above them with a plus sign. Click the plus sign to add them to the battle. The unit must be moved from that hex the stack stands on to add another unit from the stack.
If a unit or stack is outside the battle area, then you can move them into the battle area....1 hex in. Click on the battle then exit, then click on the stack and move into into the edge of the battle. You should then be able to use the above method to add the units into the battle itself.
(Granted, this assumes that you have the "Organized Warfare" technology, which allows for reinforcements)
Many thanks. I didn't have that technology, hence no reinforcements!