how to send files to the nas?

First you have to decide how you want to use your ReadyNAS. You can use it as a file server at which it does a very admirable job. In this way you can access a file from any computer on your home network at any time without having to have more than one PC turned on. Or you can use it to back up the files on your computer, and it does a very good job at this as well, although it is a bit easier if you use some third party backup applications.

For general file server use, to get the files up on the NAS, you need to set up your shares the way you want them. The default shares are backup and media. Of the two I only use media and have other shares such as finance (for bank records, worksheets, copies of documents, etc) and operational (for everything else except video and pictures which are in media). Then just as any PC drive organization, you can copy the files up with copy/paste. (In Windows, either the Computer or Windows Explorer windows works fine).

If you have Windows applications that use certain file storage locations as a default, you may want to map a share to a drive letter. For example I have my finance share mapped as drive letter O: so that when I open Excell, it automatically points to my finance share (which it knows as drive O:). (My mapped drive letters are O:, P:, Q: and R:) Almost any Windows application can be set to a new default (other than C:) by going into the application and changing the default file location.

Just expanding on PapaBear's post. There's a few how-to's here:

Windows: http://www.readynas.com/?p=127
Mac OSX: http://www.readynas.com/?p=4907