Q:

How do I enable a greyed out datasheet button?

On my list the datasheet button is greyed out. How do I go about enabling it?

A:

Just on the off chance: You are using Internet Explorer, right?
If not, I'm afraid this is an IE-specific function; as I discovered when trying to pry it open with Chrome's dev tools.

A:

As SPDoctor stated - it sounds like an issue with 64 bit install of Office.
You cannot view a list in Datasheet view after you install the 64-bit version of Office 2010
Try installing the 2007 Office System Driver: Data Connectivity Components

A:

Do you have the Microsoft Office client software (32-bit version) installed on your computer? This is necessary to support this function. You may also need to enable ActiveX controls if this is disabled in your browser.

