How to solder SMDs on metal core PCB?
A good DIY method for the occasional "home reflow solderer" is the electric frying pan technique. It allows you to heat the MCPCB very evenly. Almost any $19,99 model from a thrift shop will do the trick.

First, clean the MCPCB with Isopropyl alcohol. Apply a bit of flux with a flux pen. Then squirt a minute bead of solder paste, use one that already comes syringed, on the anode, cathode and heat-sink contacts of the MCPCB. Next place your LED in the right location. Turn on the electric frying plan until the solder paste liquifies. You will see that the surface tension will suck your LED in the right location.

With this technique, you can even "home-reflow" intimidating looking DFN ICs and the like surprisingly accurately. Sparkfun et al. have good examples of this in their old video library here.