Can't rename files

I use an application called iBank. I downloaded an upgrade file (all their files are iBank#-Update.dmg, where # is some version number). I like to rename them by changing "Update" to the download date. That way I have a chronological order of what was downloaded when, in case I need to go back. When I try to rename a pop-up reports "You don't have permission to rename "iBank#-Update.dmg". More info showed read-only but I changed permissions to read/write. This is some security setting on my new iMac. On my old Macbook (both running SL) I can rename.

Yes, I am the administrator (only account).

Figured it out. I have a root level folder called "downloads", with a folder per application I download. Within each folder I stick all the upgrades that come along, renaming each with a version number and date I downloaded. Somehow on this new iMac the permissions were set at the root level to read only. I reset "documents" to read/write. That did not change the permissions on the subfolders. I changed the iBank subfolder permissions to read/write and that allowed me to rename the just-added upgrade. All is well. Except I don't want to manually change permissions on all those subfolders within "downloads". Is there any way to bulk change the permissions of all those subfolders?

I noticed for the subfolders in "downloads" there are three names under Sharing and Permissions:
_unknown
admin
everyone

The root level folders, except "downloads" all show three names
system
admin
everyone

"Downloads" has three names
Myusername(me)
admin
everyone

I set this iMac up with a single account (admin with Myusername). There is no guest account, as guest is grayed out. Who is _unknown? BTW, I used migration assistant with an ethernet cable to do the migration from the Macbook. I am thinking _unknown is my Macbook username (I changed it when setting up iMac, can't recall if I was forced to or did it for fun).

Doesn't chmod require folder-by-folder permission changing?