Have you lived there long enough to compare the performance of the system to previous summers? (Sometimes units are undersized, or paired with poor duct design.)
Is the air coming out of the vents cold? Is it as cold as you remember before problems started? Is the airflow at the vents good?
After he added coolant, is the system working better? If yes, the system was low on charge. If not, something else is wrong.
It is possible for the system to be low on charge, but still be able to keep up on days that are moderate (albeit at a lower efficiency than a properly charged system).
It is also possible for there to be a leak, but the HVAC guy missed it. He probably only did a quick leak check (soap bubbles). There are more involved (expensive) tests he can do. However, they may not be necessary. If adding coolant made the system work well, wait and see. If the performance degrades again in a few weeks/months, you have a leak.
If there is a leak, it is usually in the outdoor unit, which is exposed to the elements, and takes a beating.
The good news is that the unit is probably still under parts warranty (typically 5 years for a cheap unit, 10 years for a good one), so if there is something expensive wrong, the parts should be covered.
(When was the last time your air filter was replaced? If it's been more than 6mo, look at it and see if it is OK. Replace if necessary. If you aren't up for replacing your air filter at least once a year, you should consider a service contract where the techs do it for you.)
Did the problem just start? Is the condensation on the pipe frozen? If so, this is an indication that the compressor is overworked (possibly low on coolant). If you are seeing any frost, you should turn off the system before the compressor burns out and call an HVAC professional.
Insulation is wrapped around the coolant line to prevent condensation from forming and causing damage. That insulation may have started to fail. If so, replacing it is an easy enough job.
I reccomend you turn off the system for a little while so the coolant line can warm up. Pull off as much of the old insulation as you can and wipe it down to remove any leftover condensation. Then replace it with the tightest-fitting and highest R-value insulation you can find.
When re-insulating, be sure to seal all of of the crevices. If the pipe insulation tubes have an open slit in the side, make sure you seal the slits with insulated tape. If any copper is exposed, it is a point for condensation to form. The coolant line should be completely covered from end to end.
I reccomend that you don't bend the pipe. The coolant pressure is somewhat higher than your generic water supply, and if you damage/crack the line, it won't be pretty.
Best Answer
If your unit is producing cool air, the evaporator is probably working. The condensation usually falls into a drip pan located under the evaporator heat exchanger. The drip pan drain can become clogged, which will drop the water into the air handler and from there, gravity takes over. Gravity started it, but once it's out of the pan, it's anywhere.
Start with locating the drip pan and checking the drain. You may have to sop up the condensation from the pan with sponges, towels, absorbent cats, etc. You can check with a stiff wire to see that the drain is clear.
The exit for the drain can become clogged. My drain dripped onto weeds which grew upward into the PVC pipe, allowing slime to grow into the pipe. It became a plug which had to be removed using a shop vac (wet/dry vac). Once removed, I was able to open the top of the entry point and add some diluted chlorine bleach, repeated on a regular schedule.
I also re-configured the drain to enable it to drip into a section of large diameter PVC filled with pea gravel, reducing the growth in the future.
Newcomb and company has a useful set of instructions similar to mine above.