Recall that you can easily search inside Newzie thru search bar located at the top of the window, as shown in the screenshot below. Other than that, you can also use search dialog which enables you to tune your search by giving more options to set.
To open search dialog, just click that magnifying glass shown at the right of the search bar.
After clicking that icon, search dialog pops up, shown below. At this mode, search dialog only allows you to enter the search query and set 'Match case' and 'Match whole word' switches.
If you click 'Options' button, dialog window expands and two other settings emerge. As shown below, first option enables you set time interval in which you like to search, while the second one allows you to define your search space by giving you the chance to select specific subscriptions and containers to search inside them only.
When you click 'Search' button, Newzie starts to search and you can monitor progress of that search thru search bar. You can even cancel the search while in progress, thanks to the 'cancel' button, which is only visible during searching process. Following sample screenshot shows search bar while searching is in progress.