Adding details to @erik's answer above. Version 3.0 of the API only seems to return plans for effective dates greater than or equal to 01/01/2015. To get plans for 2014, you must still use Version 2.0 of the API here: https://finder.healthcare.gov/#services/version_2_0 (and marked as deprecated). Also it seems the effective date cannot be more than 150 days in the future.
In response to the question about the plan data not matching what is on healthcare.gov, please note that this API only returns Private Plans that are NOT in the Marketplace. The website finder.healthcare.gov uses the same API and tries to explain that on the home page here: https://finder.healthcare.gov/
Edit: sdwarwick below just pointed to this: https://www.healthcare.gov/health-plan-information-2015/ which is where you will find plans that are in the Marketplace. Stand Alone Dental Plans from the Marketplace are here: https://www.healthcare.gov/dental-plan-information-2015/