ACR should never visibly open going from LR to PS. This is by design.
Because your LR and CS/ACR are mismatched versions, LR will have to render to a TIF on the way into PS, so there’s no ACR involved. If you were using matched versions of LR and PS, either LR2/CS4 or LR3/CS5 (or now LR4/CS6) then ACR is called on the way into PS but without your seeing the user-interface. The adjustments available in LR3 are “newer” than the adjustments available in CS4/ACR5 so it doesn’t even make sense you’d want to see the sliders in ACR on the way into PS, because they’d be undoing or changing something LR had done unless you mean you want to adjust the intermediate TIF file using the older ACR sliders for some reason.
Wow, that's not easy, but I think, I understood. But... (sorry for that)
When I open the pictures in PS, I do not want the adjustments of LR. I use either LR or PS. So, I always open the CR2 and do not render LR adjustments into TIFF. That's why I thought, PS should give me the ACR UI.
As you’ve discovered, LR’s not made to do what you want—have ACR come up on the way into Photoshop with the starting of Photoshop initiated by LR. In “Adobe’s mind”, LR and ACR have the same adjustments so there’s no reason to have ACR come up and have thing be edited outside of LR and LR not know anything about it, and overwrite those adjustments the next time you use Edit In with the same photo.
What is the goal of your wanting to use ACR not LR to adjust a photo you’ve initiated the editing of from within LR?