If the game year is before 3071, heavy lasers do not appear in megamek lab, even though the weapon was introduced in 2405. They should be available starting from 2405.
http://i.imgur.com/xGvNgw9.png
P.S. If possible, can someone rename it to "Support Laser (Heavy)"? That's the actual name on page 301 of the tech manual and the discrepancy may be confusing.
I'll fix this in MM.
OK....major rework on Infantry weapons. Names match TM now. MegaMek commit [r11679]