Hi @dinbogdan 
 
The method isNewsletterEnabled, doesn't exists in all blocks, only in two as far as I can see doing a quick search of the core:
 
- Mage_Customer_Block_Form_Register
- Mage_Customer_Block_Account_Dashboard_Info
Meaning that you can't just call it from any template file, either you can extend your block from these, or you can use the following code to replicate it: 
 
 
Mage::helper('core')->isModuleOutputEnabled('Mage_Newsletter') 
Hope this helps :-)