Display your NowThen pictures on your widget sidebar or in a gallery page.
Download the plugin archive and expand it into the directory wp-content/plugins/nowthen-photo-display (you've likely already done this).
The archive comes with an empty folder nowthen-photo-display/cache. Make sure this directory is writable by the web server by using chown/chmod.
Go to the Plugins page in your WordPress Administration area and click 'Activate' for NowThen Photo Display.
Go to your Plugin settings > NowThen and specify your NowThen username. (If this step is skipped, the default NowThen public picture feed will be displayed)
Go into your Widgets management screen and add the NowThen Photo Display widget to your sidebar.
To add a NowThen picture gallery, create a new page, add "[[nowthen]]" to the page's content, and publish the page.
Congratulations, you've just installed NowThen Photo Display.