I've had the same issue before. I got a way to make it work tho..
Assuming your server provider provides you with a File Manager in the control panel, browse to:
server/server_identity/cfg
(Replace server_identity with your own, if you're not sure, there'll likely be just 1 folder anyway)
In the cfg folder create a file called users.cfg
Add the following line, containing your own ID and name:
ownerid [steamid] "yourname" "no reason"
According to http://steamidconverter.com/http%3A%...%2Faiekillu%2F, your line would be like:
ownerid 76561198044932670 "Aiekillu" "no reason"
(No idea with "no reason" does, the server added this after a restart, I entered it without.)
Edit: The source where I found the solution is in the link provided in the forum post mentioned above. To be precise: http://rustdev.facepunchstudios.com/dedicated-server under section 'Moderating'
To define someone as an admin, or a moderator on your server simply type into the server console
moderatorid 76561197960279927 garry
or
ownerid 76561197960279927 garry
Where the number is the 64bit steamid of the person you want to add, and the name is the name of the user. The name is only used for your reference - it can be anything you want.
People added using ownerid will be able to define other moderators and owners. That's the only difference between the two right now.
Next time you join the server you will see that you have been made an admin, and you can send console commands to the server (F1) just like you were sitting at the console. You will also be able to use admin commands (like inventory.give).
You will find as admin that by default you get some special abilities.. like instant/free craft. At the moment these things are mandatory - but we will endeavour to make them optional in the future.
Your users are stored in <identity>/cfg/users.cfg