My guess is that exec command is not correctly formed. It should look like this:
<exec program="C:\Program Files (x86)\Microsoft SDKs\Windows\v8.0A\bin\NETFX 4.0 Tools\sn.exe" verbose="false">
<arg value="-i"/>
<arg value="${rootFolder}\ApplicationFolder\MYKey.pfx" />
</exec>
You can also look at examples here as reference