Here's some instructions from FreshBooks on the API fields and where to find them:

http://community.freshbooks.com/support/faq/what_is_my_authentication_token_and_where_can_i_get_it/

You'll want to input your admin or staff API information in the Setup→My Personal Information→Personal Information page.

This error means that your FreshBooks API URL you have entered in the Setup→My Personal Information→Personal Information is not valid. Try copying and pasting the URL exactly as it appears in FreshBooks and try your request again.

This error means that the FreshBooks Authentication Token you have entered in the Setup→My Personal Information→Personal Information is not valid. Try copying and pasting the Token exactly as it appears in FreshBooks and try your request again. If the FreshBooks password has changed recently, the updated Authentication Token must be copied into Salesforce.

Salesforce imposes restrictions on the number of custom applications, tabs, fields and functionality set available in each edition. That being said, the Connector is designed to work with any edition including Group and Professional.

However, the Connector does count against certain limits in all editions, summarized below.

If you've reached any of these limits, then you'll need to correct them before Salesforce will let you install the Connector. For example, if you want to run the Connector in a Professional Edition with 5 applications already installed then you'll get an error until you remove at least one.