Problems with combining objects

[mclc]

Hello,

I know it's strange, but I cannot combine objects together in my inventory. I take the string, bring it to the bottle of baxtrin, and my character basically tells me I'm an idiot for thinking about combining these stuff.
It's the same thing for the empty lighter and the fuel.
I didn't know where to put this, as I may have missed how to properly combine those objects (I doubt it, I even went to see a walktrough just to confirm that I was combining the good things), and since I'm on a mac, maybe it could be a problem?

Thank you very much.