Just as the subject suggests, in a custom tab an action button is only visible to super users. It appears to be an access problem. The question is, what object should I be looking for to rectify this problem?
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Never mind. Reviewed my own comments and figured it out.
While I see that you were able to find the answer on your own, I simply wanted to remind you that questions written to your own Discussions space (as seen in the upper-left: All People > irubalski > Discussions) don't get the same amount of attention as those posted directly to the appropriate forum.
You can still consider moving this as-is (no need to recreate) to the proper forum for maximum visibility. You can do so by selecting "Move" under ACTIONS along the upper-right. Then search for: "Documentum" and select the first result that appears. This will relocate it to the Developers Network subspace of the Documentum community.