Strange Error in shipping admin when clicking on UPS & USPS

I keep getting a strange error screen when I try to click on/install the USPS or UPS modules in the shipping area in admin. Error reads:

Forbidden
You don't have permission to access /admin/modules.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Any ideas? I have consulted a few other Zen people and they have sent me here. Permission is set to 644 - but I think it is supposed to be that way. Have tried to reinstall the php files via FTP, but it still doesn't ever show the "install" button.. . just gives that error. (Running Zen Cart 1.3.6)

Thanks in advance.

when I try to click on/install the USPS or UPS module
and
but it still doesn't ever show the "install" button.
This is confusing?? first you state that you click the install andthen later you state thta you have no install button???

As you are not very far - install 1.3.7

Sorry.. clairification:

I am attempting to install the modules. Both appear in the admin, but when I click on the item to bring up the "INSTALL" button, all I get is this white error screen with that verbage.

I am holding off upgrading to the next version as I am afraid it may break a few additional mods I have installed like Super Orders and an Image Handler.

still puzzled as to why we are getting the error...