You'll need more than two wires to make this work
The first fact that we need to take into account here is that your boiler produces actual, live steam, not merely hot water, and thus its controls need to be treated with more care than normal. In particular, blindly wiring R/Rc on your Nest to the R terminal from the transformer will bypass the low-water cutoff on your boiler, which is vital for safe boiler operation. (You do not want to test your safety valve the hard way, because on a good day, that leads to a really lousy sauna experience in your steam-laden basement.)
Furthermore, blindly powering your Nest from the existing thermostat wires can cause it to lose its brain whenever the low-water cutoff kicks in; this is aggravating, to say the least, and may not be an acceptable state of affairs in some systems. As a result of this, the "power connector" Nest ships with is something we'll have to ignore. (It's for the 99% of folks who have forced-air systems.)
So, you'll need to replace your existing thermostat wire with a cable with at least 4 wires in it. Once that's done, you can then install the Nest, wiring it as follows:
- Red -- R on the boiler transformer (the right hand terminal) to Rc on the Nest (very important that this goes to Rc, not Rh!!!)
- Green -- green wire in the low-water cutoff cable (that's currently connected to the thermostat wiring) to Rh on the Nest
- White -- black wire on the pressuretrol (that's also currently connected to the thermostat cable) to W on the Nest
- Yellow/Blue -- C on the boiler transformer (the left hand terminal) to C on the Nest
This causes the Nest to treat your system as a two-transformer system, with Rc and C powering the thermostat at all times, yet with W on the Nest connected to Rh during calls for heat, thus giving you proper system operation provided the Nest is configured correctly, with "Radiator" for a system type and "True Radiant" mode enabled. This is necessary because steam systems have a very strong "flywheel" effect due to all the stuff (boiler, pipes, radiators, rooms) that needs to heat up in order to get heat to the thermostat.
That characteristic also means you shouldn't rely on deep daily setbacks or regular use of the Nest's Home/Away function to try to save energy. Instead, you should treat your system much the same as a variable-speed type heat pump, setting it for a reasonably comfortable temperature and leaving it set there unless you are going on an extended vacation.