The Eisenhower is said to look like "a space suit on a diet" and the Donavan with its flowing cloak of IR and radar absorbing mesh seems like an anime hero of some sort. However, the best short description of the MacArthur I've found is "like a six-year-old trying to draw a robot from a 1950's movie".
This is actually a fairly appropriate and accurate appraisal. Its torso is large and boxy, the head a sensor turret that looks like an inverted fishbowl made of smoked glass (actually, a strong polymer), its hands fairly crude manipulators, its legs and feet over-sized, and a hump on its lower back. Squared off pauldrons cover the shoulders and upper arms - acting as a mounting point for over the shoulder launchers and rabbit-ear antennas wich beak up the otherwise smooth lines. The legs are thick, with feet described as "duck-like" to help distribute the units rather considerable weight.
It is hardly surprising that the unit gamed the nickname "Might Mac" from its physical presence. Most armors add a few centimeters and kilos - but the Mac adds some 50cm and 60 kg. Indeed, it is often thought of as a single person armored fighting vehicle rather than worn armor.
Most TASC units use standard infantry carried weapons, but the armor's ability means their weight and recoil is far less noticeable, while sensors improve effectiveness. The MacArthur carries heavy infantry support weapons to start with, and goes up from there.
Fifty caliber machineguns are some of the lighter loads used by the armor, though semi-automatic weapons of that caliber are more common, since the sensors and recoil systems for for accurate, yet rapidly mobile sniping. (Even with other armors, .50 sniping is best done prone) 20mm cannons are also popular, though these usually have a very low rate of fire to off set limited ammunition payload, and the still considerable recoil and bulk of the weapon.
"Boom Tubes" are a common choice for anti-armor work. Much like a "roman candle" firework, they are a series of charges in line along a tube. However, each is an electrically actuated shell, and the pilot can select the number of rounds fired with a simple switch. Smaller versions in 40mm are general purpose, while 65mm or 88mm tend to be used with shaped charge or thermobaric rounds for anti-vehicle and structure use.
Of course, the main reason for the existence of the SMAC is to carry weapons too big for augmented humans. Up to four tube launched missiles can be attached to the unit's back, each containing a 180 kilogram missile of either anti-aircraft or anti-tank variety (or more often, the new multi-purpose multi-fuse missiles that can do both jobs). A second super heavy option is a breech loaded 90mm motor, with revolver shell feed. Somewhat less common is over the shoulder nine-tube rocket pods containing 70mm rockets, similar to those used by attack helicopters.
Although the Mighty Mac is a bit ponderous, it is quite strong in a melee fight. However, despite what modern media would have you believe - there are no purpose built power armor swords, maces, flails or other melee weapons. There really isn't much point, as fighting another armor hand to hand would be anything but stealthy with the sound of motors and crunching hard armor. An Ax exists - meant for utility purposes, but does quite nicely, and an entrenching tool/shovel is sometimes used as well. Indeed, due to the potential damage to the armor's manipulators, any weapon, improvised or not, is preferable to a simple punch. The mechanics of the balance systems generally prevent kicks from being delivered as well.