on the back side of each 5.25" bay cover (the black plastic piece attached to the front bezel) you should find a bracket to attach drives. you'll find photos here: http://forum.lenovo.com/t5/ThinkStation ... true#M2561
if the eSATA port is the newer "I" style (which it should be) then you can try finding an eSATA to SATA cable online. the difference between that port and the other three is likely the ability to hot-swap. internal eSATA ports are somewhat rare and i'm surprised the M90p has one internal and not on the back instead.
regarding drive power, any of the spare 5-wire SATA power plugs will work for either a HDD or ODD. the 280W PSU is more than enough to power two HDDs and two ODDs with everything running at 100%.