Whatever version of LW you're using, you can only preview as much of the file as you've downloaded. LW connects you to other users for downloading - there's no central server. So you have to download at least part of a file from another user to be able to preview it

If you're new to LW, check out the user guide & FAQ here: LimeWire FAQ and User Guide

And also have a look at the sticky threads (in red) at the top of different forum sections here