Very easy to find on a map. It's at the Southeast corner of where Interstate 90 meets Interstate 5. It's called Dr Rizal Park.
It's a bit more difficult to drive to from the freeway, bring a GPS or at least a good city map. Not bad really, just lots of city streets and you need to find the right one.
Steve's shot is from the bridge where 12th Ave S crosses I-90. It will be quite obvious when you get there.
Turk and my shots are from near the parking area at the park. Directly opposite the Pac Med building, on the west side of 12th Ave. Just follow 12th up the hill a bit, and look for the parking on the right side if you're heading south.
A quick look at a Google map will make it all pretty clear. As mentioned, I suggest using reasonable care in that part of town, especially after dark. Then again, I've never been bothered, or even approached, but I have seen some scruffy looking folks around.
Here's a very nice view of the location:
http://jazno.net/recentwork/wp-content/uploads/2008/05/35-amazon-headquarters.jpg
You've probably seen this building heading South on I-5 just after passing through downtown.
The green bridge on the left is a nice location, decent views and it has a pretty safe feel to it. I typically shoot from near the South end of the bridge, I like the view from there best. It's curved on that end, and I walk just far enough to the North that the bridge isn't in the shot. Steve's shot gives you a good idea of the view from that location. Turk's shot from the bridge looks like he shoots from about the same place I do.
The other location would be near the right edge of the photo, in the park. It's adjacent to a parking area, and has a very nice view. Turk's photo is a great example of what you can get from there. Both locations are a short walk from one another, so park in the lot and try out both of them.