The spell description says that you can make a melee touch attack against a weapon held by an enemy in order to affect it with the spell. However I can't find rules anywhere about making such an attack, only the sunder rules (using CMB), or rules for attacking unattended objects. Does anyone know? Thanks
Make a house rule
The rules don't cover this precisely. Actually hitting a weapon in an attempt to deal the weapon enough damage to destroy it is generally done using the combat maneuver sunder, but this is different (needing only a touch) and the spell rusting grasp adds its own specific rules, however incomplete. Making touch attacks against creatures and unattended objects is precisely covered by the rules, but making a touch attack against an opponent's weapon—an attended object that the opponent's usually trying to shove into your face—isn't. Mechanically, all that's said about making such an attempt is that—according to the spell rusting grasp, anyway—the rusting grasper provokes an attack of opportunity from the weapon's wielder.
I suggest the following.
House Rule: Touching a Foe's Weapon
To touch a weapon an opponent wields, the attacker makes a touch attack against the opponent except that doing so provokes an attack of opportunity from the opponent and that the size bonus or penalty to Armor Class is based on the size of the opponent's weapon instead of the size of the opponent.
Note that this topic in general is rarely discussed, although it pops up occasionally when some wacky dude tries to use a spell creatively, such as in this 2011 Paizo messageboard thread that begins with a discussion of using of the spell shrink item on a foe's weapon that ends up using rusting grasp as precedent.
On the efficacy of rusting grasp generally
Keep in mind that the 4th-level druid spell rusting grasp usually just isn't a very good combat spell, mainly because "[m]agic items made of metal are immune to this spell." Thus by the time a level 7 druid can cast rusting grasp, if facing a lone foe, that foe'll likely be armed with a magic weapon and wearing magic armor, and, if facing multiple foes, it's a needless expenditure of resources to use multiple 4th-level rusting grasps to destroy their nonmagical armor and weapons when the same character could use, for example, the 4th-level spell flame strike and burninate all his foes or the special ability wild shape to assume the form of tiger and claw the crap out of them.
Niche uses notwithstanding, while rusting grasp's effects are unique—it's ability to instantly destroy a nonmagical metal weapon with no saving throw and no SR is interesting—, most of the time in a combat situation a druid's better off with a good, old-fashioned flame strike.