Using Velocity tools in a QuestionForm on Mechanical Turk

I'm writing a QuestionForm XML in order to create HITs for mechanical turk. According to the documentation, velocity templates are supported and can be used in the xml, but I can't get velocity tools to work.

I am able to use variable names that take their input from a test.input file, for example, <Text>$aaa</Text> works if the input file contains the field aaa.

However, I can't use the velocity EscapeTool, so this doesn't work:

#set( $bbb = $esc.url($aaa) )
<Text>$bbb</Text>

In the log file velocity.log I get an error that the RHS of the set command is null. I get the same error if I just do #set( $bbb = $esc ) so this means that the EscapeTool is not recognized.

The EscapeTool documentation here says that this segment should be added to a configuration file:

<tool>
  <key>esc</key>
  <scope>application</scope>
  <class>org.apache.velocity.tools.generic.EscapeTool</class>
</tool>

But I'm not sure where this configuration file is. If I need to create it, how do I point to it?