The new avocado tree that I bought isn't growing well since I planted it in the ground. Should I put more soil around it to build the ground up?
No, don't do that; that's something you might do as a temporary thing at the onset of winter in cold areas, to protect the roots, but not piling it up against the trunk of the tree.
Knowing where you are would be very useful, but generally, avocado trees, though not fussy about soil ph, do require free draining soil, they do not like having 'wet feet'. At the same time, any new tree requires water during drier spells for its first couple of years, and in the case of avocado, the recommended minimal amount is 1 inch per week, so it'll need a couple of gallons applied every few days, but if it's really hot, more frequently than that. As the tree grows more roots over a year or two, the best way to irrigate then is to give it more water but less often during hot dry weather, to encourage deeper rooting. If your weather has been dry, and you've not been watering as often as suggested here, that might be the problem.
If your soil is heavy, you can make a 'mound' by piling up soil in the area, about 2 or 3 feet high by 5 feet wide, and then plant into that, which will improve drainage round the roots. Also best to add a couple of inches of organic mulch (bark or similar), though again, a good 6 inches clear all round of the trunk base. If this is the case with you, provided the tree hasn't been in longer than a few weeks, you could dig it up, build the mound and then replant, but this isn't necessary if your soil is light.