First you have to make sure that paging is enabled on your main bar. Usually the main bar by default is bar one, but to be sure click the General tab. Once there unlock the bars, and look at your main bar that you use for your primary skills. There should now be a number written on top of the bar, this number is the ID for that bar.
Once you have the number for your main bar, lock them back up and click the paging tab. Make sure that the same number as your main bar is checked, in fact it should be the only thing checked unless there's something else in the priest layout that uses paging I'm not aware of. Once you have this done, check to make sure it's working by holding shift (with your mouse away from the bongos options window) and scroll the mouse wheel. When you hold shift and scroll the wheel it should flip between the pages you just made. Your main bar should be the only thing changing. If it looks like your main bar is just disappearing, that's normal because you most likely don't have the show empty buttons option enabled. Right click on the mazzleUI entry on FuBar to enable it there.
After you have the pages set up, click the stances tab, then click the Add button near the bottom. At the top of the new window click the dropdown box, and find the stance that you want to use. Then just under that you have to tell bongos which bar you want to switch, and by how many pages. If your main bar is bar one, then we'll put in this box 1-1, with a 0 in the pages category. Keep the page skip bar towards the bottom at 0. Then just click accept and you're done.
If on the other hand the drop down box on the stances tab is empty, or you can't find anything like Shadowform in there, then it isn't an actual stance recognized by blizzard and there isn't a way to completely automate it because of the 2.0 patch that nerfed a lot of mods. There is a workaround however by using a macro.
You'll have to go through all of the paging setup as before to make your main bar have an extra page that we can put spells in, but you don't have to mess with the stances tab. Instead, make a new macro with the same icon as the original shadowform spell. Add this into the text box:
Code:
/swapactionbar 1 2
/cast [actionbar:1] Shadowform; [actionbar:2] Shadowform
You can't use active buffs as a variable in macros anymore, but you can use your bars now. What this will do is detect which bar you're using, and then cast an appropriate spell. So if you're using actionbar 1 (your main bar) while you're casting holy spells, then when you use this macro it will cast shadowform for you and switch to bar two, which you'll fill up with all of the shadow spells you want to use in shadowform. Then when you use the macro again it will cast shadowform once more (which should hopefully cancel shadowform, but I'm not positive on this one) and switch back to bar one with your holy spells.
Hope this helps.