Samsung Galaxy S — APN PROBLEMS please help

I've been trying to set my mommas APN settings on her galaxy s4 (straight talk now it was a Verizon plan before) anyhow. I've found her apn set up page, but I can not add a new setting with the little plus sign at the top. It's grey and I can't get it to change for nothing. I've tried changing the old apn setting but it will not let me reset her mmsc (which is set to not set) it's grey and won't do anything when I touch it. I've also read on here where you tap the build a number button until it developes, it is now turned on and still no change with her apn...I have no idea what to do now. Her mms come through but very slowly sometimes a day later or longer. And fb is slow.