How To Set Multicast Rate On Apple AirPort Extreme

I have an Apple AirPort Extreme Rev C (2011) and I need to set a manual multicast rate. While I can set a manual multicast rate, I only have the option between low, medium and high.

In the previous versions of the Apple AirPort Extreme you had the option of 1mbps, 2mbps, 4mbps, 6mbps and 12mbps.

How do you get these options back? I need to set the multicast rate at 6mbps, not a vague range of low, medium and high...

For simultaneous dual-band APs, the AirPort Utility displays Low/Medium/High instead of numeric rates in the multicast rate pop-up menu because it means different things in the different bands.

Here's what those settings mean, per band:

   Low:  2   mbps in 2.4GHz,  6 mbps in 5GHz  
Medium:  5.5 mbps in 2.4GHz, 12 mbps in 5GHz  
  High: 11   mbps in 2.4GHz, 24 mbps in 5GHz  

Note that the AirPort Extreme's default "Automatic" radio mode supports 802.11b clients in 2.4GHz, so the multicast rates for that band in this table are 802.11b CCK and 802.11-1997 DSSS D[B|Q]PSK rates. In contrast, 802.11 in 5GHz started with 802.11a OFDM rates, so there are no 1/2/5.5/11 rates in that band.

If you change your radio mode on the 2.4GHz radio to G-only or N-only, the 2.4GHz band "Low/Med/High" multicast rates match the 5GHz values in the chart above. Note that to see some of these options, you'll need to hold option on the Mac or alt on Windows while clicking the Radio Mode pop-up menu. Also note that "N-only" requires clients to be N-capable to join, but it doesn't force clients to use only N-style MCS-based rates; it still enables the A/G OFDM rates so that clients can choose those rates for possibly greater range or power savings.

Contrary to what you said in your Question, there was never an option for a 4mbps multicast rate because there is no such thing as a 4mbps data rate in 802.11, a, b, g or n.