If button is disabled you should choose a vocabulary first, to which you want to copy entries. To do this:
If there are no vocabularies in that list, you should create at least one to have an opportunity to copy entries there:
You will be asked whether to use more than 255 symbols for the translation field (the dictionary will be larger) or not (it's more efficient, but the size of the translation field will be limited).
Now button is enabled and you can copy entries from the current vocabulary to the chosen one.
Note: hold the mouse cursor over this button to see the name of the vocabulary, where the entries will be copied.