How to connect a 3 wire plug to a 2 wire outlet?
Disclosure: I'm not an electrician, but I know a few and do my own electrical work to be inspected by more qualified people.

Generally in the manual for such appliances they advise grounding the green wire to the electrical box you would be installing the fan in.   

This doesn't really gain much of the benefit of having a ground wire available, as it most likely will not be running to a proper ground.  Regardless, in older houses where ground wiring isn't available most of the time (mine is like this) that's the best you can do.

That said, you may want to consider if possible replacing an upstream outlet near this device with a gfci outlet.  It doesn't require a ground wire, but you can set up ground fault protection for this device without it.  I've been told that is one way to get grounded (eg. 3 prong) outlets in a house with older wiring.   You would have a circuitbreaker -> gfci -> 3 prong outlet configuration.   Then the 3 prong outlet is protected by the gfci.

This might be useful reading as well:
https://www.thisoldhouse.com/ideas/replacing-two-prong-receptacles