On my mobile browser it is close to the top, on the right side and looks like a magnifying glass. Or there is also a drop down menu on the left that gives a "search forums" option.
Correct you would click on the magnifying glass then the search box will come up. In black highlights it will say Search in gray text if I'm not mistaken. You will type your search words on here right over the text search. You can also click to check search by titles only. I would recommend you not to check this to give you a more broader search parameters. Typing in the section right below it is to search by members name(I think member search section is in white)
That's just it i do not have any type of icon to click on for search. I belong to many forums so I am familiar with the format of them. Ok I did find it in the drop down menu, i had a senior moment lol.