I've got it worked already,thanks to rhedd-5. I just had not gotten the time to put a reply here.
the proper procduers are:
1.add a 'SHIELDED' behavior to the land behavior list (important so that the game reads the values for the shield)
2. add a value for the <Shield_Points> tag
3 add a value foe the <Shield_Refresh_Rate> tag
4. add an armor type <Shield_Armor_Type> tag (usually this line will not exist in originally non-shielded units)
Note: the shield_armor_type works like the armor_type tag, therefore u can put in any armor type as long as it exists in the gameconstant.xml but not necessarily those designed for shields.
This is a very good experience for me ( a modding newbie). I did not read the behavior tag carefully before and this experiece solved many problems that i had with new features on units not working.