Latexian includes commands in the View menu to show a Preview or Live Preview for a document. By default, these commands are also available in the window toolbar. If you don't see these commands, it likely means that you don't have a MacTeX or BasicTeX installation on your computer. You can confirm this by opening Latexian's Preferences and looking at the "Preview" section. Under the "Folder for TeX Installation" setting, you should see green checkmarks next to all of the commands (with the exception of makeglossaries if you are using BasicTeX). If you see red exclamations, you either need to set the Folder for TeX Installation correctly or install MacTeX or BasicTeX.
The Preview and Export functionality in Latexian requires that you have the MacTeX or BasicTeX (2009 or later) distribution installed. Both of these can be downloaded from the
MacTeX website. After installing MacTeX or BasicTeX, you will need to quit and re-open Latexian for the toolbar Preview commands to appear.