Photo Assist is a discontinued product. It was discontinued before the release of Dreamweaver CS4, as such it was never updated to be compatible with Dreamweaver CS4, it cannot be used in that environment.
I just tried downloading Site Import 2 and did not have a problem. You mention that it "unzips as WS_SiteImport212.mxp.html", you should not need to unzip it. Once downloaded to your computer, the file name should be "WS_SiteImport212.mxp". When downloading are you selecting the option to save the file to your computer? We have recently released a video that shows the proper steps for downloading an extension here:
perhaps this may help.
Power Store is not an extesnion, you do not6 install it using the extesnion manager. It is a collection of PHP pages that you just extract to your site, then run in a browser. Please see the PowerStore Getting started guide for installation instructions:
When setting up your site, Dreamweaver has a Testing Server tab, and a Remote tab. Set up the remote tab to connect to your remote site so you can use Dreamweaver to sync your remote site files in the same way you sync the local testing server site files.
When you switch the servers, you will need to reconfigure the remote tab in the site settings to connect to the new server. you will likely have to modify your database connections as well.