Start Rule

I want my Activity to start after 1 hr of starting of the Process. But how do I know when my Process has started so that I can mention it in Start Rule GUI

The activity will not be hit until the process itself has started. If this activity directly follows the start activity, you can set a start rule on the activity to start 1 hour after planning. Once the process gets to your activity, it will effectively wait for one hour before continuing. Otherwise, place the process start time in a datafield and set your start rule to run 1 hour after the datafield.

Start Rules can only be used to mention a TimeFrame alone. Does it mean that if I want to start my Activity after a Process/Activity level Flag is Set, it cannot be done by Start Rule?

You can use a preceding rule to determine whether an activity should start based on the value of a datafield. Think of it this way: preceding rules check IF the activity may start and start rules will check WHEN the activity may start.