Hey there, A Furrry Peanut.
Basically it should be compatible with a SATA III hard drive as the SATA interface is backwards compatible. It doesn't matter if a docking station, external enclosure, or a SATA to USB adapter is SATA I, II or III. Any drive with a SATA interface should work properly. The difference is that it will be limited to the speed of SATA II if that's the dock's interface. On the other hand, if you're talking about a hard drive, then you have nothing to worry about, as no HDD can reach the SATA II maximum speed and it won't be bottlenecked even in this case.
I can't really tell you if you've connected it or not, you could try a drive to see if it's recognized or simply open the case and check that out. For the SATA interface - check the case's specification, or contact the manufacturer's customer support and ask about that.
Hope that helps. Please let me know how it goes.
Boogieman_WD