Microsoft Visual Studio 2010 does not support the Compact Framework.
At the launch Microsoft made statements that it would come later, however over time that message has changed to no support in Visual Studio 2010 at all. Microsoft has been very silent on why and there has been some uproar on MSDN regarding this.
But whatever the reasons the bad news is that the Compact Framework, which the FlexPendant runs on, will not be supported in Visual Studio 2010 as was hoped. http://msdn.microsoft.com/en-us/library/sa69he4t.aspx
This means that for FlexPendant SDK development you need to use Visual Studio 2008 Proffessional edition, which includes and supports the Compact Framework.
Now Microsoft Visual Studio 2008 Proffessional edition is still being sold seperately, however if you have already purchased Microsoft Visual Studio 2010 you can contact your local Microsoft Support to get help to downgrade to Visual Studio 2008. If so please make sure to state that you need the downgrade for development on the Compact Framework.
ABB had representatives there. Amongst other things we (and other big names) tried to get Microsoft to respond to what will happen with Compact Framework. But Microsoft chose not to respond to any questions on the topic.
This means that there is still no official plans from Microsoft regarding Compact Framework.
Thus FlexPendant SDK development will still be on Visual Studio 2008 proffessional only.
(With the added complication that RobotStudio SDK 5.14.x is now upgraded to .NET 4.0 and is therefore only supported on Visual Studio 2010, so someone who needs to do both is recommended to have both Visual Studios).