http://help.squarespace.com/customer/portal/articles/733499#create-a-kit-on-typekit
USING TYPEKIT WITH SQUARESPACE
Typekit is a third-party service that allows for using custom fonts directly within your site. Squarespace integrates 67 fonts from Typekit directly within the Style editor to allow for using these within your site.
These fonts can be located when editing elements within in the Typography area of your Style editor options.
Squarespace automatically creates and updates the Typekit ID created for your site to include your Squarespace domain and all custom domains you have linked to your site.
Note: If you change your site name for your Squarespace site, there may be a delay in the Typekit fonts appearing at your Squarespace URL.
If you would like to use additional fonts via Typekit, you can create a Typekit account and add your own Typekit ID using the steps below.
Note: The built-in Typekit fonts use a Typekit API that allows for the font to appear instantly when visitors access your site. If you use your own Typekit ID instead, visitors may see a momentary delay while the Typekit font loads on your site.
Step 1 - Create a Kit on TypekitBack to top
NOTE: If you have not already signed up for a Typekit account, you can do so here.
Name your kit and add your domains for your Squarespace site, then click the Continue button.
Note: Add all of the domains associated with your Squarespace site to ensure the fonts appear regardless of the domain used to access your site. This should include the versions of your any custom domain both with and without the WWW in front.
Copy the provided code from Typekit and click the Continue button.
Step 2 - Add your Typekit ID to your SiteBack to top
Login to your Squarespace 6 site and click the icon to enter the Settings area. Select the General tab in the left hand menu.
Paste your Typekit ID into the Typekit Kit ID area, then click the Save Settings button to save your changes.
Note: Once you have added your own Typekit ID, this will replace the kit that is added to your site by default.
Step 3 - Add Fonts to your Kit on TypekitBack to top
Browse for fonts in the Typekit library. Select the font you would like to use and click the +Add to Kit button.
Note: Typekit does not allow the uploading of custom fonts from other sources for its retail plan customers. That feature is reserved for Enterprise plan customers only. Add the CSS selectors to apply each font you've added to your kit and click Publish to apply the Typekit fonts to your site.
Note: Locating the CSS selectors where you want to add the Typekit font requires programming knowledge, as well as the use of developer tools. Squarespace does not provide support for adding or modifying code within your site. For more information on how to use custom CSS code in your site you can reference our guide here.
Step 4 - Use your Typekit FontBack to top
Once you have added fonts to your Typekit, you can then use these within your site in place of the default Typekit fonts.
Note: The use of third-party services are advanced modifications that are NOT supported by Squarespace. Typekit has a great Support FAQ here.
Note: If you add your own fonts from Typekit, the default Typekit fonts will be replaced by the fonts in your own Typekit.