Make a page about your silverlight app, describe the functionality and let people know why they should use it. This makes it easier for both google and humans to find your app.
If you actually want the content of your application to be crawlable, then it's going to be that easy. Silverlight SEO seems like a great place to start, there is a lot of information on how you could overcome some of the potential SEO issues of using silverlight.
In case of silverlight only being a plugin on part of the page, then content on that page, should reflect the content in your plugin.
Also keep in mind that Pagerank is a combination of your sites content and how many people reference your site. If the content isn't visible to search engines, you will need to spend more time on word of mouth or social networks.