Question: I am using netsuite builder for creating a website. I want to create a mysql table in it but it does not allow me to do so. Please suggest how to create custom mysql table in netsuite database.

Answer: 
In the admin role, go to the menu
Customization>List,Records & Fields>Record Types> New

Give it a name and ID, alongwith any other options. Save the record. Once the record has been saved, it gives you the option to add fields to the record.

Note: this would be a NetSuite table or to be precise - Custom Record Type.
If you have access to the NetSuite account, you can create custom records.