Once you have logged in, you should see a "vehicle's" tab along the toolbar. If you do not see this tab, it is likely that you haven't got it installed.
To install the feature, click on the "Features" tab and scroll to "vehicle checks", you will then be prompted to install - click install.
You should now see the "vehicles" tab on your toolbar. If you still have issues afterwards, let the support team know by raising a support request here: https://mymobileworkers.zendesk.com/hc/en-gb/requests/new