First make sure that port is listening (telnet to it: telnet ip.addr.of.sql 1433). if it doesn't, go to HKEY_LOCAL_MACHINE\Software\Microsoft\MSSqlserver\MSSqlServer in your registry (HKLM\Software\Microsoft\Microsoft SQL Server\InstanceName\MSSQLServer\LoginMode for named instance) and change LoginMode hex value to 2 (both windows and sql login allowed).

Somtimes enterprise manager can't connect to sql server, then you need to create alias with client network tools and connect to that alias.