By default, the Datasheet view of a list or library is not supported
when you are using the 64-bit version of Microsoft Office 2010 installed
on a 64-bit Windows operating system.
If you want to use Datasheet view, we recommend that you install the
32-bit version of Microsoft Office on a 64-bit Windows operating system.
your business needs require using the 64-bit version of Office 2010,
you can install the 2007 Office System Driver: Data Connectivity
Components to enable the Datasheet component. These components are
available here on the Microsoft download center:
2007 Office System Driver: Data Connectivity Components
For more information, please see the full MS article here: http://office.microsoft.com/en-us/sharepoint-foundation-help/use-datasheet-view-in-64-bit-office-2010-HA101882420.aspx