I cannot add any subdomains or a records on my server.
i can go to domain.com but not to sub.domain.com
and if i add an new A record it doesn't work
Can anybody help me please.
DNS takes time to work for you and everyone on the internet, depending on their DNS servers. You can check for example on http://www.squish.net/dnscheck/ what rootservers think of your subdomain.
If you tried to reach your subdomain on your own PC, that result has been cached in your PC. To clear it: (windows): go to start -> cmd -> enter the following: ipconfig -flushdns
Now your cache has been 'flushed' - made empty, and when you try to go to your subdomain again a new DNS request will be made. If you are still not getting your website, wait another 10 minutes and flush your cache again.
If it's then still not working, but you see your IP address on that squish website, try using OpenDNS servers http://www.opendns.com/ on your pc - they pick up changes quite fast.