Textifier is a plugin file for WordPress that comes packaged as a zip file. You do not need to unzip the file, simply install it to your WordPress blog as you would any other plugin. (Plugins->Add New->Upload).
Once the Textifier Plugin is installed, look under Web Dimensions menu item in the admin section of your blog to find the Textifier admin page, and once there, validate your registration key with your purchase email to enable the plugin functionality.
If you see a "fatal error" reported by your WordPress blog when you try to activate the plugin, that means your web server is not configured with Ioncube loaders. Most reputable hosting companies nowadays have the option to enable Ioncube, check with tech support for your hosting company for how to install and enable Ioncube loaders.