There is a gross bug in Studio when it comes to alot of the Integration Service activities.
They simply dont appear in the activities pane… whats worse is how inconsistent it is if they appear or not.
Apparently you can get them to appear consistently by using the little plus button in a workflow to ‘add an activity’ which then brings down that dropdown at the top (really not my preferred method to add things) and then missing activities should appear there.
Its pretty bad this bug has been open this long and that its not documented anywhere in the Integration Service documentation, however thats part of a bigger issue, I’ve been chasing them to correct incorrect documentation and add missing documentation for nearly a year now but they apparently don’t have the capacity 
Let me know if finding it via the other menu works Kevin?