Germinating Hops

I just purchased a packet of your Humulus lupulus seed and I wonder if you could explain more fully how I can best germinate these seeds here on the west coast: for outdoor sowing, do they have to overwinter before germination, in which case I would have to plant them in the fall? Or would it be best to put them in the fridge until germination - which you indicate could take up to 12 months?

Hops seeds need a cold treatment to induce germination. There are several ways to provide a cold treatment. Nature, of course, supplies 2-3 months of winter conditions in Canada to seeds that fall on the ground. Typically, the seeds germinate in the spring after the ground warms up.
In many case, hops included, seeds don't actually require freezing temperatures, just cool temperatures, in the 5 degree Celsius range, for anywhere from 1 to 3 months, or more. It is critical, however, that the seeds be moist during this cool treatment; it is not good enough to simply place the dry seeds or the seed packet in the fridge. After the cool period, the seed box is moved to a warmer area, at a temperature of 20-25 degrees Celsius.
The optimum length of the cold treatment period is not known, and it could vary from lot to lot. If you start with a 3 month treatment at 5 degrees, and then move to 20 degrees for 1-2 months, and germination has not occurred, you can put the seed box back into the fridge for another 1-3 months followed by another warm period.
As long as the seeds are hard and not mushy, you can repeat the treatment cycles several times. With hops though you should get results after the first cold treatment.
Because there is considerable latitude in the exact length of treatment periods, we generalize often by saying "1-12 months". In practice, it is best to start with a 3 month period.

I notice from the catalog that Humulus japonicus is "readily grown from seeds". Would this be a better choice? – I wish to use them to grow along a fence.

The japanese hops (H. japonica) is an annual, while the common hops (H. lupulus) is a perennial. H. lupulus will probably be the better choice because you would not have to replant it each year.