Update
Atlassian OnDemand Bamboo features a Windows stock image as of mid July 2012, see Azwandi Mohd Aris' comment:
Just an update that Bamboo OnDemand now ships with a Windows-based AMI - see release notes at https://confluence.atlassian.com/display/AOD/July+2012#July2012-Bambooupgrade
This image meanwhile also supports MSBuild as per James Dumay's answer from early November 2012.
Initial Answer
The configuration of builders is one of the Restricted Functions in Atlassian OnDemand for Bamboo:
Bamboo OnDemand only runs builds using elastic agents. Hence,
capabilities can only be configured by using a custom image.
So in principle you would need to either use one of the default Stock images provided by Atlassian or Create a custom elastic image of our own. While Atlassian provides a Windows stock image as of Bamboo 3.4, the version of Bamboo for OnDemand is most annoyingly still stuck at 3.0 (see Atlassian OnDemand Application Versions), with the long overdue update including DVCS support recently postponed again (see Lingbo Lu's comment: We are hoping that we can deliver it in July/August timeframe)
So for the time being I'm afraid you'll need to apply the Bamboo 3.0 version of Creating a Custom Elastic Image and create an appropriate custom image for the task at hand yourself.
Update
There are a couple of related threads on Atlassian Answers as well:
James Dumay's answer to How do I run .NET builds on JIRA Studio summarizes the state of affairs and recommends Grace Mollison's instructions for Setting up a windows AMI for use with Elastic Bamboo in turn. However, RodH's comment regarding How can I run Bamboo OnDemand with MSBuild? indicates unspecified issues with this approach/solution.
So be prepared for a rough ride eventually.