What is trans-addition?
Just like @QuantumAMERICCINO has pointed out, it's probably anti addition.

Furthermore, you seem to wonder why a syn compound doesn't form. It all lies in the mechanism of bromination:

The double bond in cis-2-butene polarises the $\ce{Br-Br}$ bond in a nearby $\ce{Br2}$ molecule. A $\ce{Br+}$ ion attacks the double bond and forms a bromonium ion.



The bromonium ion makes the adjacent carbon atoms electrophilic. The bromide may attack the most electrophilic out of them. Here, however, they are equivalent.



You should now be able to understand how the mechanism works.