I found some info which may help (sorry for the super long post):
Depending on the specific field and object, the issue may be resolved in one or both of the following ways:
-Create a connection to the correct table, which may require finding the table using the "Single Table" option in Step 3 of the connection dialog, as well as a join.
-If the field type is defined as a "formula" in Salesforce.com, it will not import due to limitations with the Salesforce API. Tables containing the fields involved in the formula must be brought in, and a Calculated Field must be created.
The issue is usually caused by:
-The field exists in a different table than originally thought - Salesforce can sometimes be made to show fields from one table in another.
-The field is defined as a "formula" in Salesforce.com
Salesforce.com data is stored in a series of "objects", which for all intents and purposes, are just database tables. Salesforce has a series of Standard Objects (e.g. Accounts, Opportunities, and Contacts), and provides functionality to create Custom Objects.
Another aspect of Salesforce.com is that fields from one object can be displayed on another object. For example, Opportunities show a field for "Account Name", so one might reasonably expect to find a field on the Opportunity table called as such.
Instead, the Opportunity object actually has a field called "AccountID", which is a reference to a record on the Account object, which in turn contains the field "Name". A join between the Opportunity and Account tables is required to look at this combined information together. Assistance from a customer's local Salesforce.com administrator may be required to help identify the correct tables and join relationships.
I hope this helps!