http://docs.basex.org/wiki/Databases

The OPEN command opens a database, which probably unlucky for some ;-), is not a file in your file-system:

"In BaseX, a database is a pretty light-weight concept and can be compared to a collection. It contains an arbitrary number of resources, addressed by their unique database path. Resources can either be XML documents or raw files (binaries). Some information on binary data can be found on an extra page."

I can once again invite you to work through our documentation http://docs.basex.org/wiki/Getting_Started and some basic XQuery stuff: http://en.wikibooks.org/wiki/XQuery

In addition I'd suggest you start exploring XQuery within the BaseXGUI first, most people find it a lot easier to handle in the beginning.

Best

Michael

P.S. A personal remark:

Please don’t get me wrong: it is okay to ask questions, even ones that have (seemingly) trivial answers.

Yet: you are a lot more likely to get helpful answers if you leave the impression you at least tried to thoroughly read and make use of the information provided.