Re: I credited my account by $75 then applied a promo code & didn'
Most offers require AdWords accounts to meet certain criteria before credit is applied. Check the terms that came with your promotional code for details about your offer. For example offers like "Spend $25, get $50," AdWords only counts the amount you spend after redeeming the code. If you spent $25 before redeeming the code, that amount won't count towards your offer.
So by only crediting your a/c by $75 would not show you promo amoun tin your account. First you need to spend minimum amount (say $25 in above example) to get promo amount credited in your a/c and note that after spending min. amount it may take 5 business days to show amount in your account.
for more details go through https://support.google.com/adwordspolicy/answer/1396257?hl=en