Currently my OSCOMMERCE is version v2.2 RC2a.
I have used this add-on named 'Consolidated Log-in with Guest Checkout for 2.3.1 v1.0'.
As per it's doc. I have tried all possible steps but my problem is, in the admin panel area in the category menu options I didn't find any 'guest checkout' link to enable/disable this add-on.
How to overcome from this ?
I installed this addon in my osCommerce 2.3.1 (now 2.3.4) store several years ago. It works very well. The addon control shows up in the admin control panel in configuration->Guest Checkout.
The reason this addon probably isn't working for you is that it's for osC version 2.3.1 and you have v2.2 RC2a. You should really upgrade to 2.3.4. There are A LOT of improvements... Most importantly security updates.
If you cannot install 2.3.4 and modify it with the addons you need or modify 'Consolidated Log-in with Guest Checkout for 2.3.1 v1.0' to work with v2.2 RC2a there are lots of programmers that you can hire to can help you. I'm confident anyone that would help and cares to give the best advice would say the same thing as above... Upgrade to 2.3.4 :-)