Though, normally, you might write something akin to:
(map (fn (nums) (+ 5 (apply + nums)))
...)
It's a lot like Zak's suggestion, just noting that sometimes n is infinite. Full partial application is really powerful -- especially with the general trend in Arc to have the final argument be the most dynamic one -- and I regret that I don't really have time at the moment to make an interesting example.
You're now taking a list of lists and trying to add a number to each list. That should fail - partial evaluation or no. There's no sane way for the first expression to translate in to the second.
Seconding on the module system. It may not be the biggest concern of a young language, but it's a big part of the infrastructure that will let the language grow.