I'm not an expert on this, but apparently there's a problem with the font encoding. If you change the font in the title, e.g.:
\title{{\normalsize Do\c{c}a} is still missing the c with the cedilla}
then the cedilla appears. Ok, the result doesn't really look as you would want to have it.
The class file sigchi.cls
says that the title is type set with "18pt Helvetica (Arial) bold size". Whatever.
In any case, the first thing that worked on my computer was to comment out the following line:
\usepackage[T1]{fontenc}
So apparently T1 is not the right font encoding for Helvetica cedillas.
It is also possible to change the fontencoding locally, as explained here, in case changing the fontencoding globally for the whole document makes problems elsewhere. However, then it is necessary to change the font to the title font again:
\title{{\fontencoding{OT1}\selectfont\ttlfnt Do\c{c}a} now has the cedilla}
\author{%
\alignauthor{{\fontencoding{OT1}\selectfont\aufnt Do\c{c}a}\\
\affaddr{Insitution}\\
\email{[email protected]}}\\
}
\usepackage[T1]{fontenc}
, and if not add it to your preamble.