I am assuming you mean upload the program from your PC to PLCS.net and not upload your program from your PLC to your PC.
To upload it to PLCs.net, click on the Go Advanced button under the message box you normally type in.
Once there you will have a lot of added features, including what looks like a paper clip. Click on that and it will bring up a window and allow you to upload attachments. The rest should be self explanatory.