As you may have noticed, you can't use this property for any element you created yourself.
Here's a workaround:
string sId = "Id of the element you want to change the property of";
string sInnerHtml = "The new InnerHtml string you want to set";
string sScript = @"
var oElem = document.getElementById('" + sId + @"');
if (oElem) {
oElem.innerHTML = '" + sValue + @"'
}"
myWebBrowser.Document.InvokeScript("execScript", new Object[] { sScript, "JavaScript" });