I have an older cat who has diarrhea. My vet has said he has no problem with my giving this cat Slippery Elm in the hope it will benefit her. My question is what is the correct dosage for her and should this be daily or bi-daily or even twice a day, or what?

Can't the vet help you with that info, or the person at the health food store, or possibly a vet on line you can access through google.com ? I wouldn't even like to guess at the proper dosage seeing as it is oil and too much may compound the problem.