Spreadsheet onOpen menu from a Library

Last I checked you cannot give to a trigger functions that are not contained within the script. What you most likely need to do is to create a wrapper function in the script for your scpreadsheet. Something like this:

function doSomething() {
  myLib.doSomething();
}

EDIT: The functionality that you are looking for is currently unavailable. To achieve the desired effect you would need to specify that the function that you want to call belongs to the library and that is not allowed.

There is a feature request for this:

http://code.google.com/p/google-apps-script-issues/issues/detail?id=799

If you are interested in having this functionality, please go and vote on this issue to increase its priority.

Best,

Anton