You more than likely would only use..
u1030090_marcus as the user name. The first part would be your account ID.
What about your dbase name what does that show as?
The Dbase name has two parts. The first part is what your host has called your account.
It is your account ID. In many cases it is the cut down name of your domain.
The domain myshoelaceshop.com ID could become myshoel. You can’t change this once you host creates the account so you are stuck with it.
The second part can only be 7 characters which you can name yourself.
I would name a dbase something that means something which I would recognize later such as..
shop1 or in this case the install script suggests the word , magento. So I would call it magento.
The full dbase name then looks like this, and you would always refer to it as..
Your user name also comes in two parts.
The first part is your account name the same as above.. myshoel
The second part is simple.. maybe your own name, so it would be..
The full names would then be.
You should then click the FULL Permissions option for your user name in mySQL when asked.
It should then all be set up ready to start the magento instal.
If you are not sure what you have done, you have the option of deleting your dbase in cpanel, then starting again as suggested above.
If in the Cpanel mySQL Wizard, when asked for your dbase and username just enter the LAST PART. magento for the dbase then marcus for the user.
(The program will add them to the fist part myshoel_)
At the moment it may seem painful however once you get it right (and be pedantic about it), it will all be fine.
I have tried to make it very clear although still could be confusing if you are very new to it. however I hope that it is helpful to you.
Did you check with your host support about the localhost part. It is usually localhost however maybe your host has set his server up differently. Always check with the hosts before you try to fix something to be sure you are doing it correctly and not wasting time.