I think the simple answer is that personal weapons will not do any damage at all to WarMeks.
If your GM really wants to have infantry do damage to a WarMek or other hard armoured target with personal weapons, then the conversion rate for damage is 1 point of WarMek scale damage = 10 points of personal scale hit point damage. This is mainly included for when WarMeks fire at soft targets, but I suppose it could work the other way. Remember, though, that hardness still applies, and that the damage reduction is 1/10th of your WarMek's armour for WarMek scale damage- so the full armour is applied against personal weapons.
Therefore, while a WarMek with an armour of 21 on a location is hit by a WarMek scale weapon, it reduces the damage by 2- but against infantry fire, it will use the full score of 21 as damage reduction. After this reduction is applied, the hit will still need to do at least 10 points of damage to reduce the armour by one. A miniumum damage of 31 would be needed. This is why you can generally rule that a WarMek is immune to Infantry fire.
The AntiMek Missile is the one exception to this. With an average damage of 44, it could chip away at the 21-point armour plate in the example above. Personally, I'd save dice rolling and rule it does 1d4 or 1d6 WarMek scale damage, depending on how much of a threat your GM wants Infantry to be.
However, not every Infantryman, or even every Infantry Squad, is going to have an AntiMek Missile. I'd say that an average Infantry Platoon would have one, thus there would be an AntiMek Missile for every 3-4 Squads of Infantry. A Heavy Weapons Squad, trained and equipped for AntiMek operations, would have more- at least one per squad, perhaps even two.
As you can see, WarMeks are not meant to be brought down with hordes of infantry. They could certainly threaten a lone Scout WarMek if it was caught alone by sufficient numbers, but otherwise, Infantry are just something to waste ammo on.
A better use for Infantry in WarMek combat is in a support role. Infantry on foot are very hard to detect, but can still carry comms gear- a cheaper and more effective alternative to recon drones. A Scout WarMek may be invisible to other WarMeks and their sensors, but to infantry it's still fifteen feet tall and ten tons in weight. And while WarMeks may be immune to infantry fire, their support vehicles are not.
So, whilst WarMeks will not fall to a swarm of Infantry, the foot soldier can still be a threat. Instead of using AntiMek missiles to openly attack a WarMek company, they ambush the lone scout that is operating several kilometres ahead of the main group, or lie low as they go past then pick off the robotic followers. And as this is happening, they are using spotters hidden in the woods nearby to call in artillery strikes and report on their movements to their allied WarMeks or Tanks.
And all this is without my having read the Infantry sourcebook for A:2089 (the name escapes me- the Soldier's Guide?), which could well elaborate on everything I've said here. Your GM probably needs to buy this if he wants to use Infantry a lot.