Adding anything like seeds to M&P is indeed a tricky business -
People have put their base in the refrigerator until almost set or simply waited for it to cool but you run into the risk of it stiffening before you're done.:cry:
There are a couple of options: you can pour one layer, wait for it to harden, add your seeds (spray with alcohol first and after to make sure your layers stick together) and pour your second layer - this will also create a pretty cool visual effect, especially if you do multiple layers. (again, alcohol between each layer as the 'glue')
The other way is to get a suspending base, formulated specifically for that purpose.
Good luck! :wink: