Before you can use your imp, you need to commission it. See http://devwiki.electricimp.com/doku.php?id=commissioning for instructions. Note, you have done step 1 in the commissioning document already by working ...
You may have noticed I switched browsers (Firefox to Crome). The operation is the same in both these browsers. Note that at time of writing this article, Internet Explorer does not work with the planner.
You will notice your Impee has a button top right and a link bottom right. Click on the top right button and a dialog box will open with a drop down field. Click on the down arrow to open up the list.
You can see a list of various firmware both public and your own created firmware. Under ‘Your firmware’ you will see ‘Wireless Thermistor’ which is the firmware you have just created. Select it. You will see the name of your Impee change to ‘WIRELESS THERMISTOR’ and soon after that, you will start seeing the message ‘Sent: xx.x” (xx.x is some number in degrees).