Nothing....ACR doesn't 'open' in Lightroom. Most (all?) of the tools in the Develop module are based on the same processing engine that ACR uses, but because they're built in there's no need to open a separate instance of ACR.
Unless you mean that you can't see any of the Develop tools in the right-hand column of the Develop Module?
Thanks Jim that clears things up. I was expecting ACR to open like it does in PSE and then to process the file into a PSD or TIFF. So am I right in saying in LR3 it stays a NEF until you finally export it to some destination and then you choose the format conversion? My confusion came from LR3 showing the file name extension as NEF+jpeg so it wasn't clear which I was modifying.
Thanks for your help.
If LR is showing the filename as NEF+Jpeg, that simply means that you have set your camera to shoot Raw+Jpeg and that you have the option to "Treat JPEG files next to Raw files as separate photos" unchecked (General Tab of the LR Preferences). With that option unchecked, Lightroom imports the Raw only of the 'pair', the Jpeg half is merely noted in the metadata section as a "Jpeg Sidecar". There's nothing you can do with that Jpeg in Lightroom, but any Operating System action you have LR make on the Raw (e.g. rename, move, delete) will also be applied to the Jpeg "sidecar". All other processing within LR is applied only to the Raw.
With that option checked, LR will see and allow you to import both parts of the pair which can then be processed as independant files.