My husband is a federal employee (law enforcement ) in Puerto Rico for 2015 (whole year). He is exempted of paying federal taxes. Where we can enter this information?

Recommend you review Tax Topic 901 which specifies US govt employees in Puerto Rico must include their wages on a US tax return.  Quote from TT901:

"However, if you are a bona fide resident of Puerto Rico and a U.S. government employee, you must file a U.S. income tax return reporting all income received for performing services for the U.S. government, including services performed in Puerto Rico as a U.S. government employee. If you are a member of the U.S. Armed Forces or a civilian spouse of an active duty member of the U.S. Armed Forces, special income tax filing rules may apply to you. For more information, please refer to Publication 570 and Notice 2012-41."

However, Pub 570, Chapter 3 indicates he needs to file a Puerto Rico tax return in addition, pay taxes, then obtain a foreign tax credit on his US1040 for taxes paid to Puerto Rico.  From Pub 570: 

"U.S. Government employees.   Wages and cost-of-living allowances paid by the U.S. Government (or one of its agencies) for working in Puerto Rico are subject to Puerto Rico tax. However, the cost-of-living allowances are excluded from Puerto Rico gross income up to the amount exempt from U.S. tax. In order to claim this exclusion, you must:

See:

https://www.irs.gov/taxtopics/tc901.html

https://www.irs.gov/publications/p570/ch03.html#en_US_2014_publink1000221281

The "foreign tax credit is available in order to avoid double taxation" is the crucial information here. How can this be accomplished in TurboTax? I need to do this as well. I am a US Federal Govt employee stationed in Puerto Rico, and Puerto Rico "state" tax is withheld from my pay and shows up on my W2 in boxes 15-17. I have heard that to "avoid double taxation," Form 1116 should be used and the entire amount of tax withheld by Puerto Rico will show up on Line 48 of Form 1040. How can this be accomplished in TurboTax?

You will enter this information in a Form 2555 (Foreign Earned Income Exclusion) or Form 2555-EZ is simplified version of this form.

Here's how to add – or remove – a Form 2555 or 2555-EZ from your return:

1. Open your return click search and type in foreign income (upper right corner).
2. Click Jump to foreign income in the search results. (see attached image)
3. Continue through the foreign income section, answering questions as you go, until you reach the screen Do you want to try and exclude your foreign earned income?
4. Answer Yes (to add a 2555 or 2555-EZ – we'll figure out which one you need) click Continue, and follow any on-screen instructions.

Tip: The -T or -S suffix after the 2555 stands for taxpayer or spouse, respectively.