The Intel(R) Desktop Board DH55HC will support only VT-x that is a feature of the processor. The VT-d needs to be supported by the motherboard and in your case, the H55 Express chipset wont.
Victor, thank you for confirming my suspicion.
<rant>I is quite lamentable that Intel decided to cripple the features of its motherboards.
DMAR units are located in the north bridge/CPU packages, and the only support
that motherboard should provide is coming from BIOS, i.e. initialization and correct
DMAR table filling. There is no technical reason for this feature inaccessibility.</rant>