landshark
HMFIC
Exactly. Too many people telling OP not to use them without knowing the levels of chlorine or chloramine. If others don't need to use campden tablets, that is great. Some people do and it is needed. Chlorine and chloramine can cause off flavors. Some people have very high levels of chlorine and chloramine in their water. A 50 cent tablet each brew is a very cheap fix for that.
It's been stated, but it's worth restating: the only thing that determines whether or not an additive is needed is a Water Report, and not someone else's arbitrary advice or personal experience. OP needs to get a report from his/her municipality and based upon that, determine for him/herself whether an additive is warranted.
However, the OP asked if he could salvage the beer (an Irish Red). The response was simply a campden tablet is not required to the mash to try to salvage. Water reports and chemistry is a much more detailed science than a beginners brew section needs to get involved in. For the sake of a beginner or a rusty brewer, water chemistry is not something that should be considered unless their own water is undrinkable as just brewing won't cover most off flavors.
It's brewing beer, you can be as detailed as you want, or as care free as you want. However, just because a campden tablet was not added is not going to ruin a beer.
Yooper answered the second part of the question with it can not be added to the mash.
Water chemistry has it's own section in the threads.