The Canon 6D needs a higher version ACR (Adobe Camera Raw) than what is built into CS4 while Lr5 has the latest ACR so it can open the Raw files.
You have the option of
a) either upgrading to CS6, or
b) converting the Raw images to DNGs which can be opened by CS4. For this you can download the DNG converter from Adobe's website free of charge.
You can find information about CS6 by entering (Photoshop CS6) in the search box at the top right of the http://www.adobe.com/ homepage.
CS6 requires OSX 10.6.8+ on a 64-bit multi-core Intel processor.
As suggested above, using the DNG Converter is an option:
I don't know Macs so can't give you advice if CS6 will run on your Mac.
But since you have Lr5 you won't need to open Raw images in Photoshop. Lr5 gives you all the Raw editing capabilities that you need. I don't think CS6 will add anything to Raw editing capabilities that Lr5 doesn't have - although I don't know CS6 since I am still working with CS5.
You still can send images from Lr5 to CS4 for editing but you have to select <Render using Lightroom> in the dialog that says that the "Camera raw plug-in 8.1 may be required ...". BTW: what this means is that the Camera raw plug-in 8.1 is required for Photoshop, not for Lr.
When you select <Render using Lightroom> Lr will create a TIFF / PSD from the Raw image and send it to CS4 for editing.
If you had CS6 Lr would send the Raw image to CS6 but on saving the edits in CS6 a TIFF / PSD would be created.
The only drawback with using CS4 is that the options <Merge to Panorama ...>, <Merge to HDR ...>, and <Open as layers> will be greyed-out and are not available.
If instead of selecting <Render using Lightroom> in the dialog box you would select <Open anyway>, nothing would happen. CS4 would open but the image would not be displayed in CS4.