How can I use a font that is not available on Marketo?

Is this for a Marketo landing page you are hosting on your site or an email? 

I will need to use this font both for landing page and the email "save the Date" and invitation. Basically all our communicaitons will need to use this font.

You will probably need a subscription service like fonts.com that allows you to embed non-standard fonts on your website.  Google Fonts (which is free) is also a possibility, but they don't provide Arial Narrow.  There's a good chance you'd be able to find something similar, though.  I use GF for our own landing pages and it's super simple.

There is also the option of uploading your font to your website and coding the stylesheets yourself:
http://www.techrepublic.com/blog/web-designer/embed-custom-fonts-on-your-website-with-the-font-face-property/

That said, I've been able to do this with websites, but haven't tested it with email.  With all the additional coding and CSS, many email providers may strip out the formatting or mark it as spam (my biggest fear!)

Email support will be limited at best. Use a service like TypeKit for your landing pages.

Please be cautious adopting non-standard fonts. I agree with Jason Long, it is better to avoid for email.

Web-safe fonts can be rendered by any device, any browser, any screen esolution. While distributing a PDF document is quite immune to those details, it is important to keep in mind web design must cater for the unknown. Loosing a small percentage of the audience because the fonts do not render properly is an avoidable risk.

For landing pages, you should just link your TTF file in the design studio.  For Emails, the above answers are pretty much spot on--I wouldn't try non-standard fonts in emails.