SQL Enterprise Manager tries, by default, to connect via Named Pipes. As this cannot run across the internet you need to "convince" it to use TCP/IP.
To do this, you can use the following proceedure.
On your computer go to CONTROL PANEL and then start the ODBC
Create a new system DSN of type SQL SERVER
Follow the prompts to enter your connection information for your
Make sure that you click on the CLIENT CONFIGURATION button and ensure that the connection type is TCP/IP and that the port is set to 1433
Follow the connection through to it's conclusion and save it.
Now try to connect with Enterprise Manager again. You should find that it is now OK.
Ensure that you are using the latest service pack for SQL Enterprise
Manager, SP3a at the time of writing and that you are using the latest version of MDAC.