Did you try logging in with the account you use for the app platform website? By default it is your account that is the admin for the app, this is a so called app cloud user account. MxAdmin is not used in a sandbox deployment.
Try to add /openid/login to your url. This should open the Mendix SSO login form. Use your MxID to login and you should be able to use the app.
Ok, I am not the sandbox expert, but with a normal cloud slot you have to set your MxAdmin password also there. Did you check your project / deploy / details setting in the cloud. At the right hand side of a normal cloud slot you can there set your mxadmin password.
Thanks for your answer. Unfortunately sandboxes don't have this option.
I realize this is an old post, but obviously it's still an issue, so:
In my experience, you can get access as one of the demo users once you run your app with production security on the cloud. (This, btw., is also useful in case your firewall or another application blocks the localhost:8080, which means you can only run the app from the cloud - or some other way I haven't figured out yet)
Go to Project Security, select the Demo users tab, make sure "Enable demo users" is set to Yes, double click the user name you wish to test, make sure the correct role is selected and click the "Copy password to clipboard" link. Then log in on your app using the e.g. demo_administrator user and the PW you just copied to your clipboard.