Thank you for contacting us with this issue.
Currently, The Welkin Suite IDE doesn't support the work with managed packages. This is why your objects from managed package are absent in the list.
Adding the support for managed packages is present in our development backlog, however, right now I cannot tell you when exactly this will be done. We consider and look for the best ways to implement this functionality.
I will keep you updated about the progress.
If you have any other questions or suggestions, please feel free to write us. We will be happy to assist.