I think you are not seeing or understanding the need for exactness.
Whether or not the Watchdog reset can be used with your board depends on what you actually have.
More precisely what AVR chip is on your board and what bootloader is installed in that chip.
And if what you have doesn't work, depending on what you have (board, IDE, and ISP programmer),
you can use the IDE or tools that come with the IDE to modify/correct (update)
a bootloader that won't work with watchdog reset to a new bootloader that will.
The information you provided above still does not answer the basic questions of what you actually have
with respect to either s/w or h/w.
Ok, so you "like" the "mega", but is that what you have? And if so, which "mega"?
There is an Arduino "mega" that uses 1280 and one that uses a 2560.
Or do you have some other AVR based Arduino?
And the s/w link you provided, actually has 27 different versions of the IDE that can
be downloaded from that page.
While the latest AVR based s/w version is at the top, that version changes through time
so depending on when you downloaded the "latest" IDE it can be different versions.
The reason that all this is very important is that in the past year there
have been some pretty big changes to the IDE, and the AVR based bootloaders
that ship with the Arduino s/w.
Some of these changes affect whether or not watchdog reset works and how to update
a bootloader that doesn't work with one that does.