Since it is not available in the database, here is the effect text of the two cards:
This creature gains Melee +1. Whenever it is attacked and receives damage you must transfer 1 of that damage to your mage instead. If this creature is destroyed you may pay 1 mana to move Joined Strength to a new target in your zone.
When this attack damages an enemy Non-Mage creature, that creature's controller loses 1 mana (if he has any to lose). If this Mage is level 5 or higher, Rod of the Arcanum rolls an additional attack die.
The wording is a little funny on Joined Strength, which may be confusing matters. Hypertechnically, it's weird to
transfer damage
instead. Previously, you could transfer damage that had been dealt, like Lifebond +X does in Druid's Treebond ability, or you could perhaps redirect an attack or its damage like Reverse Attack does, but it does seem a bit strange to use the verb "transfer" this way.
But the key, as others have said, is the word "instead". In that context, Joined Strength transfers one damage to the mage, instead of letting the attack deal it to the creature. It's not even clear to me that the rules do (or do not) consider that damage "dealt" or even caused by the attack. It may just be damage transferred from an attack to a mage.