Add the following to your CSS:
.center { width: 640px; margin-left: auto; margin-right: auto; }
The code of jqgrid has probably changed since the answer of mars 2011. With jqgrid 4.3.1, I had to add the following CSS:
.center .ui-jqgrid {
margin-left: auto;
margin-right: auto;
}
".center" finds the table's container, as written in the HTML sample of the question.