No skimmer door

I live in Florida and have lots of ants on the surface.

I have an old pool and there's no skimmer door (and no slots/holes to add one).

Some of them go into the skimmer (very few), but even those that do make it in there just sort of swirl around.

I just resurfaced and the company said with the addition of a vacuum line and a pressure-mounted skimmer door this problem would be solved.

However, I'm can't find a pressure-mounted skimmer door anywhere (and the entrance walls into the skimmer are on a 45 degree angle, so I'm not seeing how a pressure-mounted door would work). I would really appreciate any suggestions.

A weir (skimmer door) doesn't help the ants to get sucked up, what is likely happening is that your water level may be a tad too high, high water levels stop skimmers from sucking things down, they just circle endlessly above until they get water logged and sink on their own.
Also, I ripped off my weir and tossed it, it was a big PIA, and my pool seems to function better without it. Maybe the weir is supposed to help when you have excessively high water levels, but being in the country, more often than not we have excessively low water levels and the weir was impeding the flow too much causing my pump suck air.

A properly working skimmer weir door serves two purposes.

1) when the pump is on - the top of the weir creates a "dam" effect to actually draw the water and floating debris into the skimmer. The "dam" effect influences the water into the skimmer by restricting the amount of water in the skimmer as compared to the rest of the pool. This creates more suction of the water on the surface rather than below the surface.
Try this: take your garden hose and turn it on. Hold it underwater parallel to the surface about 12", facing away from you, and observe the effect on the surface water. Now move the hose closer to the surface and observe.
The surface water should be more influenced by the hose the close to the surface you get. Similarly, the water being drawn into the skimmer should be mostly surface water, thus the floating debris, bugs, etc.
If you have a weir door, next time you're cleaning out the skimmer basket, turn the pump on and ever so slightly, close the weir door without starving the water flow. You should see the pool water rushing into the skimmer, with a tremendous amount the surface debris being drawn to the skimmer, moreso than at the normal level in which the weir floats.

2) When the pump turns off - the skimmer weir door floats up to close the skimmer box. This in effect prevents the surface debris just sucked into the skimmer from backflowing into the pool.

So Matt, while I disagree with you and that the weir door DOES help the ants get sucked up, your reasoning is correct in that too high a water level will prevent the weir from working properly.

The pressure mounted weir doors are simply spring loaded shafts that pop out to wedge the base of the weir in place. The door itself will open and close with the top of the weir floating with the top of the water level to create this draw of surface water into the skimmer.

I guess one thing I did not mention is that my skimmer faces the prevailing winds (which we have a lot of all the time) so in my case, the debris seems to collect at or near the skimmer almost all of the time - maybe that's why it doesn't make too much of a difference around my place, still, my primary reason for not using it is the lower water levels which i can't really do anything about.