Lastform has a better concept: Fullscreen forms to reduce user distraction.
It works as a separate template, avoiding conflicts with themes and plugins and increasing your website performance. This is the best way to go.
However, if really need to use Lastform inside your pages, there is a workaround.
Like in other services, you can embed forms using HTML Iframes. We do this all the time with YouTube too.
You can simply add the following snippet to your page:
<iframe src="http://your-website/lastform/1" width="800" height="600" frameborder="0" style="max-width:100%;max-height:100%"></iframe>
If you want it fullscreen, use this one:
<iframe src="http://your-website/lastform/1" frameborder="0" style="position:fixed;width:100%;height:100%;z-index:999999999;left:0;top:0"></iframe>