The full format is as follows:
configure net 10.1.1.1:/tftp/config/pixconfig
is the IP address of the TFTP server, and is the path to the file on the TFTP server. You won't need if the file is stored in the root directory of where the TFTP server software is pointing.
If you're getting timeout errors then this means the PIX is not getting anything back. If it couldn't find the file you would not get a timeout error. You need to make sure you have IP connectivity and privilege's to read the file, make sure the owner/group and permissions on the file are set correctly so that your TFTP server can read it.
Other than that check the log file of your TFTP server software and see if that gives you any indication on what's going wrong.