The Day Care Man can often be very misleading. If he describes your Pokemon as not liking each other, that actually means that they will breed, but their chance of producing an Egg with each egg cycle is low. This requires further explanation if you're unfamiliar with the internal mechanics of breeding.
Each 256 steps you make, one egg cycle passes. With each egg cycle, there is a chance that the Pokemon in the day care will breed. If they are incompatible, the chance is always zero, and the Day Care Man will say something along the lines of: "the two prefer to play with other Pokémon more than with each other." He will say something different if the chance is above zero, and that's the category your Ditto and Abra fit into. His description of your Pokemon as not liking each other means that their chance of producing an egg with each egg cycle is 20%. This chance is low, but if you cycle back and forth along Route 34 and Goldenrod City, you can make each set of 256 steps go much faster. Because the chance is above zero, you will eventually get an egg.
The chance assigned to your Pokemon per egg cycle is based on their species and whether or not their original trainers are different from each other. If you pair two compatible Pokemon with the same species and a different original trainer in the day care, their chance per egg cycle will be 70%, and the day care man will say something like: "the two seem to get along very well."
If you're interested in the chances and descriptions for other combinations of species and OT, have a look at this Bulbapedia article that makes breeding a lot easier to understand.
