Are you sure you downloaded the Mac version? I think the itch.io client should pick the right version automatically. It should be a regular application bundle.
Alternatively you could try this direct download for Mac: https://mega.nz/file/8fx30CJA#6XEwNf8NZ9LQYCinNigbm_ZcIrRZxh5eLb12hnEwiFY
Hope that helps!