Could you please tell me where I find in my computer if it is R1/R2 or whatever. I went to the Dell Support, looked at the configurations and all it says is Aurora Alienware.
This is what the service Manual for Aurora and ALX shows

I have just got into the BIOS., only 16 GB of the 24 installed are recognized. What could be wrong ? Didn't I install the compatible memory? 

What "Release" Aurora ... R1, R2, R3, R4 ?

Aurora R1 is the exception with tri-channel memory. Must be installed in matching sets of 3. See PDF Service Manual for Memory Banks.

I am about to return the memory I purchased unless I receive any instructions of a way to make the BIOS recognize all 24 Gb I installed. If you or anyone knows of a way, please let me know asap.Thanks

1. Could you please tell me where I find in my computer if it is R1/R2 or whatever.   2. I went to the Dell Support, looked at the configurations and all it says is Aurora Alienware.

1. If Intel x58 chipset or Intel i7 9xx processor ... Aurora-R1.

 
2. Sounds like an Aurora-R1 (before the others were created).
 

This is what I find in the system info:

www.screencast.com/.../XMfieJLI

and in Dell's Technical Details of the Device

www.screencast.com/.../91FHrYAt

Does this info tell anything about R1 or any other?

I don't see any chipset info unless it's what appears as: Model  Core  i7  CPU x980.

It's working!. The 24 GBs of memory are now being recognized by the BIOS and Windows. How? I don't see any logic in this but so it happened. I read in one of the posts here that someone with a similar system "reseated" the modules. So I first inserted modules in slots 3-5-6. I turned it on. Saw that BIOS and windows recognized the 12 Gb. Then I turned off the computer, and inserted the other 3 modules into slots 2-4-6. Turned on computar and voila! All the 24 Gb were shown there. How this mere fact of installing the modules in two stages can make the system recognize the 6 of them, I cannot understand.

Case closed. Thank you for what you told me. Bye for now.

Also: I have just installed 3 modules of 4 Gb each of the ones I purchased  (see my first post) and they are being recognized as 4 Gb x3= 12 GB. I installed in in slots 1-3-5. Now I  have slots 2-4-6 empty but if I install the other three modules left in these slots, I guess it will again tell me it sees 24Gb installed but only 16 are available. My question is: if my computer is an R1, can it  hold 24 GB RAM, and what module sizes and types should I install  Thanks.

Core  i7  CPU x980 runs on Intel x58 Chipset

Tri-Channel memory. Memory DIMMs in matching sets of 3. Mine is DDR3-1333.