In the control panel, under "Current Files" you will find a dialog box that allows you to upload a file. In order for this to work, php will require write permissions to your uploads directory.
You can set the uploads directory to mode 777 (world write) if you like. This will allow you to upload files, but it has some security issues, you may (as I do) prefer to upload using ftp or ssh.
Various ISP's set limits on how large a file upload can be, for this reason, you may be better off uploading the file using ftp (or sftp, scp, etc..) this is usually faster anyway. Simply upload the file and it will appear in the payvex menu, select the file and you can set the price and other information, just as if you had uploaded it using a browser.
If you absolutely cannot use ftp, scp, sftp and really need to upload these files using a web browser, have a look at another of our (free) products called BasicUpload it is written in perl and therefore doesn't have the same restrictions as PHP, the upload size limit can be adjusted with BasicUpload.
Under Configuration, there is a box called "Upload Directory" fill it in with whatever directory you want (the default is ~/payvex/uploads) this should be a directory kept safely outside of web space, people shouldn't be able to access these files with a web browser.