I've just tested it our myself and it appears that you have to be editing in Advanced Mode to insert an Interactive Button (which does make sense).
If you are then try this:
Create a new aspx page in Sharepoint Deisgner and open it in Advanced Mode then select Split view (if you don't have this as the default). In the code pane I'd expect to see something like:
Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<%@ Page Language="C#" %>
<html dir="ltr" xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<meta name="WebPartPageExpansion" content="full" />
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled 1</title>
<meta name="Microsoft Theme" content="Wheat 1011, default">
</head>
<body>
<form id="form1" runat="server">
</form>
</body>
</html>
In the Code pane click at the end of the line that reads
Code:
<form id="form1" runat="server">
, this ensures that you are within the form. Your insert 'Interactive Button' button on the ribbon should now be active.
If this doesn't work then I suspect there's something wierd with your installation of SPD, I'm using Out-Of-The-Box SPD downloaded a couple of months ago, if it does then the isse may be with the page you are trying to edit
or you are not editing in Advanced Mode. I've just tested it our myself and it appears that you have to be editing in Advanced Mode to insert an Interactive Button (which does make sense).
Hope this helps.
Stephen