Login to your Simple Salon account.
In your ADMIN page, under your SMS & Email tab, tick "Enable SMS".
Select SMS Central, enter your SMS Central details (username and password) and click SAVE.
Once you've updated your deets, you'll be able to Log In.
NOTE: if your Simple Salon system is still telling you "you are not connected", make sure you check that your username and password are exactly the same as the details you used to create your SMS Central account.