EDIT: since the beta has not been updated since the 2013 AVX2 fixes were moved to the main branch, it is outdated and I removed the passwords below.
With the Steam version, right click on the game in the library, select Properties, switch to the Betas tab and enter the password <redacted>. That should unlock a branch called 'public_macbeta', which you can select from the dropdown list.
With the GOG version, in Galaxy, select the game, click More and select Settings; near the top, to the right of the 'Configure Game' heading, click on 'Beta Channels' and select 'On', then click on 'default' for the Channel, select 'Add Private Channels', enter the password <redacted>, click Submit, then re-open the Channel dropdown list and select 'Public_MacBeta'.
