The "[PAUSE;]" marker is used only in the "Quick Input" action, it is not compatible with the "Say Something with Text-To-Speech" action.
The simplest way to add a pause is to use the actual "Pause" action:
Say, 'I am?'
Pause 5,5 seconds
Say, 'How am I doing that?'
You can also use SSML:
<?xml version="1.0"?>
<speak version="1.0" xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
I am?<break time="5.5s"/>How am I doing that?
</speak>
Make sure you check the "Use Speech Synthesis Markup Language (SSML)" option.