I think the real answer is if you connect a ground, either to the incoming AC socket or to a ground rod, you are likely to trip the ground fault interrupter. There is no real 'ground' as the whole circuit is floating and not isolated from incoming AC. Make sure you have a polarized AC plug as well as reversing it will swap your neutral and hot connections.
If you mean 'where can I use as a reference point to measure from', the negative side of the bridge rectifier is a good place.
Be very careful with this circuit, it is not inherently safe and you run the risk of electrocution, even with the transformer voltage set to minimum. Connecting it to other equipment, which I guess is why you want to use the regulators, will almost certainly be unsafe. Also heed the warning about voltage fed into the regulators, even if you can find a 150V to 12V regulator, please bear in mind it will dissipate 138W of heat for each Amp you draw from it !!
Brian.