Your car is set to record in miles and gallons. You need to change that by viewing your car profile page and clicking the EDIT CAR link toward the top of the page. You can find a picture of the location of that link here.
You'll probably want to set your vehicle so you record with kilometers and litres and then click the Update Car button toward the bottom of the page.
That will take care of the units you use to record fuel-ups. There is also a way to switch the units you use to view the site. Look at the units under the logo on any page at Fuelly. You should see US, UK, or Metric selected. You can switch to the appropriate units for viewing the site by clicking one of these. Here's more info.