This is a very common question and we have several docs that discuss SFDC as a data source, but the basic answer to this question is as follows.
Before accessing the API, ensure you have one of the following editions of Salesforce, in order to use the API.
In Salesforce, this can be found under Setup > Company Profile > Company Information > Organization Edition.
You must have one of the following in order to use the API:
- Enterprise Edition
- Unlimited Edition
- Developer Edition
- Performance Edition
The API is not available by default for the following editions:
- Contact Edition
- Group Edition
- Professional Edition
(This means if your edition does not have API access by default, you must discuss your Salesforce representative about having access, there may be a fee associated with this feature)
More information can be found under the Salesforce document What Editions Have API Access? http://help.salesforce.com/HTViewSolution?id=000005140&language=en_US