Unless i’m wrong .odt is related to openoffice/writer. I have no knowledge about the .odt format, but if it’s xml you can open the file using emeditor as Yutaka already said. Then you will only see the xml code. You will not be able to open the file using emeditor and see the correct layout/document content as you will see using openoffice/writer. If you need to read/edit/convert the document you need a program that is able to read the .odt format and not a text editor.
Openoffice is free and then you should be able to read the document. Also if using MS Office 2007 there is a free add-on that will read the .odt format as well. I’m sure there is some small converters as well available, but they are most likely mean’t to convert the file into another word processing format like .doc/.docx.
gan