Just to be sure, I tested the solution of prankurjoshi and it works
As to the missing activity, I can be wrong here, but I think that you cannot install the new packages from beta for your stable release. They did a split of some activities in those packages and I think the stable release does not understand the beta ones
See here for some more context:
Dependencies per Project
For each new project there are 4 default dependencies: UIAutomation, System, Excel and Mail. UIAutomation and System packages are formerly known together as CoreActivities package. We decided to split and decouple .
I am pretty sure I tested it and I got the same activity missing on a workflow that went from Stable to Beta and then was opened in Stable again.