Dedicated FTP clients such as FileZilla can be downloaded directly from the Ubuntu software center for free, but generally Ubuntu users should be able to access FileApp on their device via FTP without needing one.
To access your FileApp files and folders is relatively straightforward:
1. Open any folder in Ubuntu
2. At the top left of the of the screen hover the mouse to get the menu (it should read File, Edit View, Go, Bookmarks, and Help)
3. Select "Go", then "Location"
4. From here, you can simply type in the FTP address in the folder you just opened.
This will make the connection and now the iPad works like any other folder, move/copy/delete files from the iPad. You can also bookmark it as well. Of course, you need to have FileApp running with FTP File Sharing enabled in order to connect and you also need to remember to put in the port with the FTP address :2121