how to: Enable third-party browser extensions?

have been getting some recent warnings when viewing/downloading pdf files in internet explorer 7/8.

this waring states that 1 or many pdf plug-ins are disabled and could conflict with viewing/printing pdf images.

I checked the addons and all are enabled.

I think i narrowed it down to enabling third-party browser extensions under advanced settings.. BUT it is grayed out when i try to enable… how do you enable it?

Thanks

xp pro sp3
IE 8 and 7
Admin rights