Previously there was no option, but to install the 32bit version. Microsoft released the 64bit version.
The following section is a repost from this site…
The 64-bit version of the directory synchronization tool is now available. The 64-bit version now uses Forefront Identity Manager 2010 as the underlying synchronization engine.
To download the 64-bit version of the directory synchronization tool from the Office 365 portal:
In the header, click Admin.
On the Admin page, in the left pane, click Users.
At the top of the Users page, click the link next to Active Directory synchronization.
Under step 4, select Windows 64-bit version, and then click Download.
To run the 64-bit version of the directory synchronization tool, you must have the following software installed:
The 64-bit edition of the Windows Server 2008 or Windows Server 2008 R2 Standard or Enterprise operating systems.
The 64-bit version of the directory synchronization tool shares exact functional parity with the existing 32-bit software. Therefore, refer to the existing directory synchronization documentation for all other deployment and configuration information.
Upgrading from 32-bit directory synchronization tool
Although the functionality of the 64-bit version of the directory synchronization tool is identical to the 32-bit version, the underlying SQL schema is different. For this reason, you cannot perform a standard upgrade. To upgrade your 32-bit installation of the directory synchronization tool, you must first uninstall it, and then install the 64-bit tool on a new computer.
Although the 32-bit instance of the directory synchronization tool is removed, the overall state of the objects in your on-premises and cloud directories, respectively, is preserved. When you install and configure your 64-bit instance of the directory synchronization tool, it finds and matches objects in the cloud with on-premises objects. However, the 64-bit instance of the tool will not find and match objects in the cloud if on-premises object deletions occurred when the 32-bit instance of the directory synchronization tool was offline.
Therefore, you must minimize changes to your on-premises objects during the upgrade to the 64-bit instance of the directory synchronization tool.
On the computer on which the Directory Synchronization tool is installed, open the Control Panel, select Add and Remove Programs, and then uninstall the Directory Synchronization tool.
|If a synchronization session is in progress, a warning message appears when you try to remove the Directory Synchronization tool. If you receive this warning, wait until synchronization is complete, and then repeat this step.|
Install the 64-bit version of the Directory Synchronization tool installation file on another computer. To do this, sign in to the Office 365 portal, click Admin in the header, click Users under Management in the left pane, click Set up in the Users pane, select Windows 64-bit version, and then click the Download button for step 4: Install and configure the Directory Synchronization tool.
On the last page of the installation program, select Start Configuration Wizard now, and then click Finish.
The Microsoft Online Services Directory Configuration Wizard starts.
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Office 365 MVP