i cant change the buff...

why cant i change the buff of the "doom" spell? . wether i remove the default buff from the the spell or add custom buff, in the game its buffs still wont change and its description always says "this unit has stricken with doom. it cannot cast spell and will take damage over time....".

First not all Buff/Ability combos can be edited. Examples. You can change the buff listed for Spiked Shell all you want, but the ability will always use the Spiked Shell buff. Pheonix fire has no buff listed in the ability, but it certainally uses the Human Phoenix Fire buff.
If this is the problem, then edit the Doom buff rather than making a new buff.

Second. If you do make a new buff then you should almost always base your new buff off of the abilities normal buff. Ie Make your NewDoom buff by baseing it off of Doom.

Third. Buffs don't _do_ anthing. Buff are just there to provide text and graphics. Adding the Entangling Root buff to a spell will never make that spell act as an Entangling Root spell.