My cats are both indoors too ! When we adopted Maya from the shelter , she was begging day and night to let her go outside ...!
Sometimes , she drove me crazy with her meowing !!
But I didn't give in !! Wayne's advise is the best , because that's what I did too ! I bought a leish and made a walk with Maya in our garden once a day . Now she is completely adjusted to her indoor life ; no more begging ! When I take that leish out , she even goes into hiding , as she does not like that "thingie" very much
I also do this with lil Inka , and she too is happy when the "walk" is over , so she can jump into her basket and have a nice long nap ...
Success !