To create a tax field for your state you would need to go here: YourDomain.com > Modules > Ecommerce: Products (mX) > Taxes

And add your state and your state's Tax amount.

(for a quick video on how to setup your taxes click here.)

See screenshot:

In the example above we are using a "US Base Tax System" for the state of "Maine" and a tax rate of 6%.

NOTE: the "State Tax" is there by default only when the tax system is set to "US"
When its set to none then it's gone and the taxes are configured manually

If you wish to manually create a Tax, you can do so by clicking the "Add Tax" button.

see screenshot:

You will want to name the tax according to your needs for the tax.

Setting the "Percent" is the base for any other calculations made in that tax.  If you need to add different percentages for different zip codes, then the "Tax Markup" will be above the percentage set in THIS area.
Apply To Items: This area will allow you to choose if this tax rate applies to only certain Items.

Apply to Shipping: This selection will determine if Shipping is taxed or not. (you can also say if you want it applied to all items or items shipped from a certain Shipping source)

Apply to Tax-Exempt Customers: This option will apply this tax rate to Tax Exempt customers.

Take Address From: You can decide if you want to charge tax on the Billing address or the Shipping address here.

Country: Choose your Country here.

State/Province: Choose your State/Province here.

Zip Code Ranges: This area is where you can specify a markup for different zip code ranges.  NOTE: it will charge a markup on the "Percentage" field specified above this option.

Apply Only to Buyers with: This area can help you make the tax apply to certain buyers in different ways.  For more detailed info on this area please feel free to contact Support