HDA isn't the hard drive activity light, it is the header for the HD front panel audio. The Hard drive activity light is among the Front Panel IO block.
The case you have bought obviously has a cable with two connectors at the end, one HDA and the other AC97.
Depending on the age, etc, of your board, you will need to plug only one of these into the front panel sound header on your board. I have often seen this header towards the back of the motherboard, and a few times to the left of the board.
The header is a 10 pin connector, with one pin missing, so you cannot possibly put it in the wrong way round.
Just going to summarize everything everyone said: AC'97 is in the simplest terms for ANALOG audio for older motherboards, while HDA is for HD Audio in the newer motherboards. Find out whether your motherboard uses HD Audio or Analog Audio. If it uses HD Audio, then you connect the HD Audio to where you motherboard is usually labeled "f_audio", althought don't take my word for it as it may be labeled different. Keep in mind this is for those who later look up AC'97, trying to figure it out where it goes.