Migrate works in such way that once you purchased premium hosting you need to request your current free account to be moved over, by submitting a support ticket in the premium clients area.
To expand on Anna's post just a little, please take a look at This Article which explains the High Resource Usage policy for free hosting users. This Link will explain the differences between Free Hosting, Illuminated Plan, and Premium Hosting .
If you wish to move to Premium Hosting simply sign up for a premium hosting account . After your premium hosting account is created you will have access to a client area where you can open a ticket and request that staff move your free hosting account to your new premium hosting account.