The documentation covers most of this.
First you have to create a search instance for that: http://wpdreams.gitbooks.io/ajax-search-pro-documentation/content/getting_started.html
Then you can access the search settings: http://wpdreams.gitbooks.io/ajax-search-pro-documentation/content/general_settings.html
As well as the multisite settings: http://wpdreams.gitbooks.io/ajax-search-pro-documentation/content/multisite_settings.htmlBest,
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