.....update, I found this answer on another post. (I may have to use third party options untill Apple provides an updated memory "pressure" usage to the dock icon.
This is no longer an option, especially since Mavericks has new memory management routines (ie, memory compression) that make the charting of memory usage less straightforward. Apple has switched to a new "Memory Pressure" approach to viewing memory usage, which is available in the Memory section of Activity Monitor, but so far has not been added as a Dock icon option. Hopefully Apple will do so, as this would perhaps be one of the more useful Dock icon options.