Not all mosfets are created equal. Many of them are meant to be controlled by 10V to 15V signals. Those types tend to work with greatly reduced performance if driven with only 5 volts, but won't work well at all with only 3.3V. And how they work with low gate drive can vary quite a lot with temperature and even between different pieces of the same part. My best guess is you might have one of those mosfets meant for 10-15 volt gate drive.
The diagram linked from msg #6 has multiple problems. Aside from using the breadboard rotated 90 degrees, the circuit has no resistors to limit the current through the LEDs. With a mosfet meant for 10 volt drive, the mosfet might be acting as a resistor of approximately the right amount to prevent damage to the LEDs if driving with only 5V from Arduino Uno, but again, the actual mosfet performance when under-driven can vary quite a lot, so that's not reliable.
Of course, this involves quite a bit of guesswork. We can help you much better if you tell us specific details, like the mosfet part number. Showing photos of how you actually connected the parts & wires is also a good path to getting much more useful help here. Don't be shy. If you look over the many threads on this forum, you'll see we have a pretty excellent history of figuring out what's wrong with breadboard-built circuits when we can actually see photos.