Text won't fade in with motion tween

you need to embed your font.

if you're using static text, the text is already embedded.  if you're using dynamic or input text, you need to embed your font.  there's nothing else that needs to be done for an alpha fade effect using text.

so, start again.  use static text, if that suits your needs.  put the text in its own layer in the frame where you want the fade to start. right click the text, click convert to symbol, click graphic (or better click movieclip).  in the properties panel, click the color dropdown, click alpha and enter 0 or move the slider to 0.  on the last frame of your fade right click the text's layer timeline and click insert keyframe.  in the properties panel, move the alpha slide to the end-fade value.  right click between the two keyframes and click create motion tween.  retest.

you're welcome.  (you could use a graphic instead of a movieclip.  that would work, too.  but there's no perceptible benefit and there could be some drawbacks.)