Click item is not available

PerformActivate sounds like the SandBar toolbar control - could you please clarify if that's the case? TestComplete doesn't provide the ClickItem method for SandBar toolbars, but feel free to submit a feature request.

As for the locking - try calling PerformActivate asynchronously. The window opened by the New button is probably a modal window, and it prevents PerformActivate from returning a value until the user closes the window - hence the locking. Asynchronous method call should help avoid the locking:

// JScript
var oResult = Runner.CallObjectMethodAsync(obj, "PerformActivate");

// Do things in the window, then close this window
...

// Wait for the PerformActivate method to return control to the test
while (! oResult.Completed)
  aqUtils.Delay(100);