The easiest way to require the company name is to show the company name field on the step 1 checkout page and require it there. Go to the cart settings under the step 1 setup section and check the box titled "Show Company Name". Then open up the step1.ascx page and add the following required field validator control after the company name textbox on line 25.
<asp:RequiredFieldValidator ID="rfvCompanyName" runat="server" ControlToValidate="txtCompanyName" Display="Dynamic"
ErrorMessage="*Required" CssClass="SmithProdText"/>