Thanks for your replies. TL;DR: It works now.
So my development environment is VmWare Player on Windows 7 + Linux Mint + FireFox + Eclipse Luna. Initially when I created a new process definition in eclipse and looked at the xml version of the diagram, it didn't contain the <process> section and all I saw was the "diagram" part. But next time after I installed eclipse in a new VM and installed STS plugin before Activiti plugin and created another process difinition, the file contained both "process" and the "diagram" sections. I think this is purely concidental and must be due to "user error". I'm just sharing my experience in case it happened to someone else.