I've had my Harmony 510 set up with my HTPC for nearly 2 years now, and saw that in the OP sleep/wake weren't complete yet.
I am using the remote as a standard keyboard basically, using the standard Windows hotkeys. In the Harmony software it is identified as 'Media Center PC'
Since the Harmony software can't send specific commands, I needed to create a work around. I remembered that you could create a shortcut icon on your desktop that sends the sleep command.
I then used AutoHotKey to map that shortcut to the F12 button. In the Harmony software the Power Off command sends the F12 button, which in turn activates the shortcut which puts the system to sleep.
To wake the PC, use the Power Toggle command within the Harmony software
Panasonic 50" ST50 1080p Plasma
Yamaha Aventage A710 / Polk Audio RM510 5.1
Logitech Harmony 510 / Logitech MX Air mouse / Lenovo N5902
Silverstone SG06 / Intel i7-3770S / XFX 7870 DD / 840 Pro 256GB / Windows 8 Pro / Synology DS413 + 4x3TB WD Red RAID5