Firstly, you need to buy an Exophone which supports incoming SMS. You need to talk to your Account Manager for this (Support widget on the bottom of your dashboard).
1) Login to your account
2) Goto the 'App Store' page and install any app by clicking on 'Try for free' . The simplest App is 'Zero'
3) Once you install the app, click on the 'Edit SMS app' link corresponding to the app that you installed
4) Drag and drop the 'Send a Reply' applet into the flow
5) Enter your application URL which will return the dymanic text in the 'Send a reply' applet's textbox
6) Save your flow and close it.
7) You can test out the flow by sendin a message to the trial number (09223183143) with the <Your-mobile-number> followed by the sms content.