The hand guard you have is a Magpul MOE... It has been replaced by the Magpul MOE MLOK... They are different products and MLOK accessories do not work with the original MOE hand guard, as you have found.

You could use something like this.
MOE Handguard Mount-N-Slot Bipod Mount V2 : IMPACT WEAPONS COMPONENTS MOE HANDGUARD MOUNT-N-SLOT BIPOD MOUNT V2 | Brownells