Integrating MVC framework to CMS (CodeIgniter)

My first advice would be to manage your client's expectations right away. Building a CMS or part of a CMS in CI requires advanced skills especially if you want your code to be used in the future and not just to solve a problem that you have at this exact moment.

My second advice in case you want to go on with this is to try several CI CMS. I would avoid the first 3 because they have very few stars on github:

https://github.com/Codefight-CMS/Codefight-CMS
https://github.com/khodakhah/nodcms
https://github.com/havok89/Hoosk

https://github.com/goFrendiAsgard/No-CMS
https://github.com/daylightstudio/FUEL-CMS

My third advice is that you follow instructions on github / CMS documentation website. If you can't get it to work try to help the developers to improve docs / detect problems you found on instalation.

My fourth advice is if you only want to do some quick stuff and you think you will never touch that again and you are not comfortable with CI, you can just go around CI and use normal PHP or other stuff. CI doesn't force you to use MVC or whatever. You can make some rambo code and move on.

I would strongly advise you to go with the first advice it's where freelancers fail most of the time.