Lightroom normally stores settings in the catalog's database, not in the .xmp file along with a raw file. If you want to use ACR to open an image that has settings from Lightroom, you'll need to save the settings to the external .xmp file for ACR to be able to read them. Under the Metadata menu, select Save Metadata to File. If you make changes in ACR those won't be read by LR so you'll have to then select Read Metadata from File to sync the changes back to LR.
But, if you are using Lightroom, why not simply open the images from Lightroom into Photoshop using the Edit In command?