Merhaba!

How do I say in Turkish I am not sure when I should use it

 or I don´t know when I should use it. 

I made up: Bunu kullanmam gerektiği zamandan emin değilim.

or Bunu kullanmam gerektiği zamanı bilmiyorum.

Are they correct?

In daily Turkish you can say;

"Bunu, ne zaman kullanmam gerektiğini bilmiyorum."

But are my sentences correct in non-daily, official Turkish?

Bunu kullanmam gerektiği gereken zamanı bilmiyorum.

O my, frankly, I am a bit confused why the construction with -dik is allowed in the first example, but it is not in the other one 

Simply explanation;

First was daily use form and second was correction of your sentence. 

Why don´t you read our gerekmek thread

         http://www.turkishclass.com/forumTitle_52635

sufler? There is some beautiful material there. Maybe it will help you.

Ok, but the problem is not with gerekmek itself this time. I mean the general rules for the construction "I don´t know when ... ". So, if I want to say I don´t know when to use it in official Turkish I should say still:

Bunu kullandığım zamanı bilmiyorum.

right?

Nope, its not right. I gave the right form and daily for on the top. 

Look at your English sentence, sufler. Indirect question, eh? Look at this thread:

         http://www.turkishclass.com/forumTitle_50979

I guess  -  and I am not sure  -  the closest translation to your sentence is the simple indirect question. I can´t read harp00n´s mind but I don´t think daily here means anything less than correct, the most accurate and easy-going. The simpler the better often works in Turkish.