Hyper-V automatically creates a MAC address pool for virtual machines usage when it is installed. Hyper-V RTM had no option in the UI for managing the MAC address pool and you had to edit the registry to manage the pool (see a detailed article on that subject here).
Hyper-V R2 has provided the ability to manage the address pool directly in the Virtual Network Manager. To manage the global MAC address pool, open up Virtual Network Manager and select the MAC Address Range option under Global Network Settings. From here you can modify the MAC address pool available on the managed Hyper-V host.
Note that making this change does not fix any existing assigned MAC addresses from the original pool. Therefore if you need to change the MAC address used or modify the range of addresses, you should make this change prior to creating the first virtual machine.
If virtual machines have already been created, then you have two options to change the MAC address.
1) Modify the MAC address assignment to static and assign a static MAC address to the virtual network adapter.
2) Remove and re add the virtual network adapter in the virtual machine and a new dynamic MAC address from the pool will be assigned.