This is the text of the power description, from Adventurer's Vault:
Power (Daily): Immediate Reaction. Use this power when an enemy makes a melee attack against you. Make a melee basic attack against that enemy, with a power bonus on your attack roll equal to this weapon’s enhancement bonus; if your result exceeds that of the attack roll against you, the enemy’s attack misses. The melee basic attack you make to block your enemy’s attack has no other effect and does not deal damage.
Say you have a Parrying Longsword +2. Then I think you get a +2 enhancement bonus and a +2 power bonus when you use the power. So you'd roll
1d20+level/2+STR+proficiency+4
Am I right?
Power (Daily): Immediate Reaction. Use this power when an enemy makes a melee attack against you. Make a melee basic attack against that enemy, with a power bonus on your attack roll equal to this weapon’s enhancement bonus; if your result exceeds that of the attack roll against you, the enemy’s attack misses. The melee basic attack you make to block your enemy’s attack has no other effect and does not deal damage.
Say you have a Parrying Longsword +2. Then I think you get a +2 enhancement bonus and a +2 power bonus when you use the power. So you'd roll
1d20+level/2+STR+proficiency+4
Am I right?