How to get deskbar-applet in Xubuntu? Or is there any alternative box where I can type for programs/files and they appear?
I've installed deskbar-applet but I don't think I can pin it to the Xubuntu panel.
Deskbar-applet appears to be very specific to gnome and gnome-panel.
As an alternative try
As you can see, it is invoked similarly via the XFCE panel or via a hotkey (defaults to CTRL+SPACE). There are a number of default plugins which are similar in functionality as deskbar as well.
It uses the Zeitgeist backend - so it is similar to Unity - when you use a file or application it is automatically indexed for you to find again.
Catfish is built in, it's just not obvious it's a file system search application. Put a shortcut to it on your panel.
I'd recommend trying Gnome Do, it's not as snappy as synapse, but the difference is marginal and it supports more apps and is much better at finding files. Also XFCE has Application Finder, very small app that allow you to find applications :)