The OP didn't say where in the Portland area that they live. "Clint" wouldn't know the difference since he is far, far away in PA. Perhaps the OP would like not only PDX, but "Portland-Hillsboro" in Washington County, or the other one in Multnomah County at "Troutdale". He might even want to find some of the smaller airports around such as Aurora.
First, go to the top of the page and click "Database", or highlight it with your cursor. Two possibilities show, choose "Frequency Database". Then click on the map of Oregon. Now you have choices. If you want the ones for Multnomah County, click on that map which takes you to the "County Home" page. At the top are tabs, click on "Other Agencies" and PDX and Troutdale appear. I would also go to Washington County and pick up Hillsboro. You'll recall the airshow is there each year and I can get much of the airborne traffic even from home here in East Vancouver. Got some good catches with the T-birds this year.
For smaller fields (and you suggested wanting to hear some of the planes going overhead) you'll need to do some research on the web. There are tons of sites with that information available to pilots, some with nearly real time radar plots of the area showing some of (by no means all) of the ones overhead. Even some aps on various Pads like iPad, with freqs and even flight rules and procedures, kind of fun to follow.
Other radio sites such as "Intercept Northwest" have good databases as well, but may not have the smaller fields as much as other sites. Your best bet for those might even be the ODOT site. Not sure, but I know WSDOT used to even publish a book with all of the fields in WA State, along with Frequencies and rules for using the field.
Don't forget picking up the tab that says "Nationwide Frequencies" and follow that to the common aircraft channels. Finally, don't forget othetr publications such as Monitoring Times, which publishes each year (usually March I believe) an issue dedicated to airshows, where tons of frequencies of all sorts are printed.
Good luck, happy listening!
Rick