Mulberry trees are wind pollinated. In some species, there are male trees, and female trees. Other species have both types of flowers on the same plant. Female flowers are small and inconspicuous. Male flowers are catkins, so long dangling things.
Are all mulberry trees dioecious?
The pfaf.org database http://www.pfaf.org/user/Plant.aspx?LatinName=Morus+nigra says nigra is monoecious. I have a 10 year old M. nigra tree that gets covered in fruits but they are only 1cm long and fall quickly. I only have the one tree as I assumed it was self fertile but I am now wondering if these are unfertilised fruit? I am not aware of any other trees in the area. Or is it just that is a young tree still?
Gail, you probably have a male mulberry tree there. If so, it will never bear fruit. It also could be a female that just hasn't flowered yet. If I were you, I'd leave it alone for a couple more years and see if it does fruit. The flowers on female trees look like small green mulberries with white hairs. They emerge with new growth in the spring all along the branches.
Female mulberries in the absence of a male will set fruit with almost no viable seeds, but they'll still fruit.
Twisted Tree Farm and Nursery