To get meantone, simply click the center note of the lattice and cycle to the scale that has only yellow, white and green notes. Then click in the upper left on "lattice" to get to "prefs". Click on "linkages" and OK the first comma, g1 = 81/80. That's it!
The white (3/2) tuning slider will be at 1/4-comma. You can move it anywhere you want, and the yellow (5/4) slider will follow. You can move the yellow slider instead, and the white one will follow. Double-clicking the white slider puts you in pythagorean tuning (which is technically a form of meantone), and double-clicking the yellow slider puts you back in quarter-comma. Third-comma has the two sliders lined up off to the left, and fifth-comma has the white one off to the left and the yellow one off to the right, by equal amounts. The fifth can range anywhere from 600¢ to 800¢. See the attached gif.
To get a specific scale, go back to the lattice page. The genchain's starting point is the green note in the lower left. The ending point is the yellow note in the upper right. The center note is the 2nd white note, which is the 6th one in the chain. To make the center note be "D" for example, right-click on the big D note. As you modulate like this, the pitch offset from A-440 will change. You can control it by clicking or right-clicking on the cents number. You can reset it by clicking on the "A-440", which also does an all sound off.