Does anyone know how to design a sloped groove such as dado joint using the Easel pro software?
Really not possible. Might try sloping the material.
Phil’s right - for the time being, Easel is purely a 2.5D design tool - you can do multiple flat layers, but no slopes or curves.
You can totally achieve what you’re wanting on the X-Carve, though, you’ll just have to resort to some other CAD/CAM software. Fusion360 is a popular choice, mostly because it’s free for hobbyists. 
depends on the slope you want. if the same slope as the vbit you have (half the listed angle for the bit), you could use the indirect method of cutting the slot with a straight endmill first, to the width of the bottom of the cut, then come back as a separate worksheet with a vbit cutting on the line of the previous pocket cut. if there are any interior corners/ends in the pocket cut you’ll need to do a small amount of hand cleanup at the bottom of the pocket (small chisel or file). It’s definitely faster than trying to do the entire pocket with a v-bit (because of the small stepover required on vbits to get a flat bottom)
you don’t actually need pro to do this if you lie to easel about the bit you are using, but you do need to set the speeds and feeds as if it were a vibit.
ETA:
for slopes that are different than the vbit you have, both of the previous posters are correct, Easel Pro won’t do it, but there are other softwares that can. Fusion360, VCarve, etc… probably recommend a ballnose bit for best wall smoothness.
ETA2:
theoretically with a lot of math you could make successive pockets of decreasing width… way too tedious to do manually, but it seems like a great idea for an app that could be added, since it’s pretty straightforward on the math.
on the subject of manual efforts, you could also go back over it with a router by hand… at least in the simpler cases
Thanks for those who answered my question, I will try to come up with another solution.
If you learn to write gcode, you’ll find that when the cutter moves two axes at once from one spot to another, that they will move simultaneously. This means you can write a few simple lines of code to move downward or upward over a given distance. You can write in Notepad and then run it in the UGS. I wrote a short file to make a 3 degree angle slot downward to guide a drill bit into a guitar neck. I drew the line in a CAD program and found the coordinates for the start and stop point.
15-20 years ago, you had to write your own code to do stuff because the software wasn’t available to do it for you yet.
You would write one line for the coodinates for the bottom of the cutter from where you want to start and another line for where you want the bottom of the cutter to end up. You can see the sloped groove here:
Basically the X value doesn’t change but the Y and Z values change over the distance desired.
Thanks for your suggestion on writing gcode. I am new to this activity so it may take awhile. Thanks.