Hi BVS,
I believe what you are looking for is actually in the Method Gmail Gadget, if you create an Activity from the gadget, when you set the dates, you will see the sliders for the time here. Unfortunately, this is not available from Method in your browser, only through the gadget.
As for your other question about changing the time slots in the Calendar to 15min instead of 30min, I believe you are referring to the Activity Calendar in Method. If this is the case, then unfortunately this is also not possible.
-Ben