If I do not have a letter in my box 12 do i just leave it blank?

If there is no letter and no amount in box 12 then just leave it blank (empty).

Do you have a deferred compensation plan or is the pension plan box marked with an X? If so, you will need a code. If you have a 401k, it would be a code D. If you have a W-2 from a prior year from the same employer, you could check that or call payroll at your company.

box 12 on my w-2 has an amount, however it does not have a designated capital letter.  How should I treat this entry?

You would have to determine what the entry is for to enter the proper code for it. You may be able to look on your last paystub and determine what it is for, or ask your employer. Once you determine what it is for look on the back of your W-2 form where they will list what codes that apply to box 12 and choose the applicable code.