Just tell her how much it will turn you on. Tell her you get so excited with just the thought, and it would ba amazing to watch her please herself, to watch her make herself cum.
Watch? Hell, I used to help her masturbate! She especially loved it when I'd put my thumb in her pussy and a finger in her ass while she played with her clit. Masturbation was like foreplay for us...
I have watched my wife masturbate several times and vice versa. She was really nervous at first but now she is cool with it. I think she thought that if she did I would get upset, but I told her that it doesn't bother me. So then I bought her a bunch of toys and stuff for her to play with. It really helped open up our sexual relationship a lot. It made us both a lot more comfortable trying new things.
Maybe she doesn't want to feel like a Zoo exhibit and have you staring at her. It makes some ppl uncomfortable and some ppl don't respond well to "requests". You got to ease her into it and do it together.
First, you got to go down on her. If you make her cum with oral, she'll be much more willing IMO. As you're down on her, include her in the experience. Try grabbing her hand slowly and make your way to her clit while you're licking away. Kiss her fingers dude. Then move your hand with hers and start the motion. Hopefully she'll keep on moving her fingers without your help after a while.
She's not a fan of oral, neither am I. She just wants to fuck god damn it. She said that's the only way. I'm gonna make this work either way but it might take some time. She just said she doesnt feel comfortable yet. It's only been like 10 days so we'll see.