Article from Netsuite Help:

https://333032.app.netsuite.com/app/help/helpcenter.nl?topic=EDIT_CUSTCENTER

 

And few screenshots from me:

 

To add new menu item to custom center/tab:

Navigate to Customization >> Centers and Tabs >> Center Categories >>

 

Find the Category / Center you want to add link to. Click Edit:

 

3. Add/Edit the link

 

You would need here a name of the suitelet deployment. 

 

Job done:

 

BR,

Alex