I've done this before with ADAT and mtc, remember it being a pain.
I don't have the system here now but you need to clock the HD24 from the digirack via ADAT.
Pluggin both the midi in and out connections (for the mtc), you should only need one but i remember there being some sort of quirkyness...
Put pro tools transport 'online'
And hit play on the HD24.
btw to check if pro tools is receiving the mtc open > Setup > Session window. Check to see if the 'incoming time' is changing.
edit.
Actually i was transferring the other way round, pro tools to the hd24. But it should be a similar setup.