Set Maximum Server Memory Higher Than What is Available to OS
Presumably you have the "Standard" edition of Windows Server 2008 R2.

You'll need to move to the Enterprise Edition (or DataCenter) of Window Server 2008 R2 to utilize more than 32GB, regardless of the "max server memory" setting in SQL Server.

Windows memory limitations can be seen on MSDN here.

Keep in mind, SQL Server also has memory limitations by edition.  SQL Server 2008 R2 Standard Edition limits memory to a maximum of 64GB.