How do I compute the average power from the PSD plot attached?
There's no simple, accurate way to do it from a graphical plot. 

The two alternatives are (a) simplest, set the RBW to 3 kHz, which will encompass essentially all the power in the signal, now read the marker 

or (b) hard work, get a numerical dump of the display, and use anything from Excel to your favourite computer language to integrate the power across the display.

However, by eyeball, the marker is at -51dBm in a 300Hz RBW. The power has dropped by 10dB by +/- 500Hz from the centre, and you get precious little extra power after that. So your total power will be slightly more than -51dBm, maybe -50dBm, maybe -49dBm.