Today I encountered this error message:
Error logging into server localhost: You must enable the Notes TCPIP port.
This appeared in the fat Lotus Notes client v7.0.3 running on Mac OS X Leopard (10.5).
To fix this issue, first of all, quit Notes. Then locate the Notes’ preference file attached to your current user. Mine was found in my home directory at /Users/kevin/Library/Preferences/Notes Preferences. At the end of this file, add these two directives:
TCPIP=TCP,0,15,0 Ports=TCPIP
Then relaunch Lotus Notes and switch from Island (Disconnected) location to another that will allow your client to listen to the network. In my case, Home (Network Dialup) did the trick:
Problem solved ! 
BTW, this error can still be encountered in Lotus Notes 8.5 on Mac OSX Snow Leopard (10.6). In this context, the solution is exactly the same.
Here is a screenshot of the location drop-down menu with the new Notes interface:
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And if you’re looking for the
notes.inifile on Windows, it’s located atC:\Documents and Settings\[username]\Local Settings\Application Data\Lotus\Notes\Data\. At least in my case with Lotus Notes 8.5.2 under Windows XP SP2: