You can't do what you want by design (at least on modern browsers). This would be a security issue if you could.
You can read more about this here.
Snipit:
Historically, the HTML File Upload
Control () has been
the source of a significant number of
information disclosure
vulnerabilities. To resolve these
issues, two changes were made to the
behavior of the control.
To block attacks that rely on
“stealing” keystrokes to
surreptitiously trick the user into
typing a local file path into the
control, the File Path edit box is now
read-only. The user must explicitly
select a file for upload using the
File Browse dialog.
IE8 read-only File Path box
Additionally, the “Include local
directory path when uploading files”
URLAction has been set to "Disable"
for the Internet Zone. This change
prevents leakage of potentially
sensitive local file-system
information to the Internet. For
instance, rather than submitting the
full path
C:\users\ericlaw\documents\secret\image.png,
Internet Explorer 8 will now submit
only the filename image.png.