Learn how to connect to the builtin WebDAV server using Finder on Mac OS X.
Your Mac must be connected to the same network as your iPad running Textastic.
The iPad must be connected via Wi-Fi.
Start the WebDAV server and note the WebDAV url.
Switch to Finder.
Open the Go menu (1) and select Connect to Server (2).
Enter the url from the Textastic WebDAV screen (1) and click on Connect (2).
Enter user name and password you chose when setting up the WebDAV server. (Defaults are "textastic" and "password"). (1)
Click on Connect (2).
You can now use Finder to manager your files.