I received an email saying I was being given a free 3 month sports mobile bundle but I can't watch any sports or find evidence of any offer in my account.
If there is nothing showing on your online NOW account with the email address that NOW sent you under My Account > My Membership.
Then has it been more than 10 days since you purchased the Day Sports Pass (assuming this 3 month freebie was included with a Day Sports Pass deal offer).
Should the Mobile Sports Pass be showing under My Membership, then make sure your using a smartphone and the NOW App from your phone Apps Store.
And you are trying to watch one of the permitted Sky Sports channels which is Premier League, Action, Arena, Racing (Horses) or News.
If going around in circles, then jump over to NOW live chat (click on the green chat online button from this link page below).