In order to search posts inside one specific subscription or container rather than in whole Newzie environment, then there are two ways you can use, as listed below:
When you open that subscription or container and let Newzie list its content in HTML-based report rather than conventional listing of posts one after another, you will see an edit field at the top of the page, right at the bottom of the page title. Simply enter the query you like to search and then press 'Return' key.
Or you can initiate search from the context menu of that subscription or container as follows:
Note that since popped up search dialog includes couple of options to tweak the search, you may like to prefer it to the first method in specific circumstances.