Home owners and house sitters are two different entities on the Mindahome website and you need to be logged in as one or the other to access the features each type of membership offers. If you are a home owner you need to log out and then register as a house sitter with a different email address.
You can open a different Gmail or Hotmail account and forward all mail going to it to your regular account - that way you can continue to only use one email address to manage your Mindahome messages.
Alternatively, if you don't plan on using your home owner account any time soon, you can log in to that account and change the email address associated with it to something else so that you can register as a house sitter with your regular email address. Just keep a record of what you changed the home owner email address to so that you can log in on that account if you need to in the future. After changing the email address make sure you "log out" of your old home owner account so that you can register as a house sitter.