Mine came with the feature; I can't even install DD-WRT if I wanted too; but it appears that most of the features it would add I already have anyway.
I accessed if by Choosing "Advanced" in the horizontal (upper) toolbar, and then selecting "Access Control" from the left- shown here:
At that point I simply select "add Policy" (as I did here for the "loger" entry)
at which point it takes me through the steps of which machines to log activity for, how to handle it (deny access and log for certain sites, any sites, etc, or allow and log access to any URL.)
Again, I don't know if this is on the DIR-615, but it's on my 655.
as for DD-WRT, one of my internet buddies was singing the praises of it some time ago, seemed to even express pity that I could run it, even though I had no intention of doing so (since I didn't need any of the features). As with any firmware flashing type of thing, it's always a good idea to proceed with caution, however.