Should I cook my pot roast on high or low?

Without any other information, it sounds like you have to assume that your slow cooker has only a warm setting (labeled "low") and a high setting, and use high for cooking. (Usually they have warm, low, and high.)

But it's possible that they're just being overly paranoid in the manual. I would personally try heating something on low (possibly just water, as a test), and seeing whether it boils or at least reaches and maintains a decent temperature.

The thing you're trying to avoid is the food spending too long in the "danger zone", below 140F (60C). If the low setting only just brings it above that, then it's going to take forever to heat up past it, and your food will be unsafe because all that time it wasn't hot enough. But on most slow cookers, the low setting brings it up to at least around 180F and on many, it even very slowly boils. If your heats that well, you can certainly use it for cooking. The one thing to keep in mind is that if it still takes a long time to come up to temperature, you may want to start it on high to get it all heated, and then switch to low for the rest of the cooking time.

With mine, "low" is what you'd use for a long slow overnight/all day style boston butt or pot roast. It runs around 200F degrees. "High" is more what you'd use for cooking in the day time, where you can come home at lunch and turn it on. It runs closer to 300F,

There will be a certain amount of warming time with the "low" setting: this is always the case when you're cooking around 200F degrees. Some people recommend starting it on "high" and then turning it down to low once it's warmed up. I don't think it really matters unless you're working with something that has already had time to build up some bacterial colonies.

One thing to remember with a slow cooker is that they tend to fail over time. If you turn it on and it heats very slowly and doesn't get very hot, it's likely your heating element has crapped out, and you need to replace your slow cooker. There is really no way to tell other than sticking a thermometer in it every now and again, but you can usually get a sense of it if you're used to using them.

The other two answers do a pretty good job of covering the pertinent issues, but here is a succinct answer:

As long as the internal temperature of your roast reaches at least 140°F within the first two hours and then stays at least that temperature for the remainder of the cooking, you are fine. The only way to test that is to use a thermometer. The longer it cooks at the lower a temperature the more collagen extraction (i.e., tenderness) you are going to get.

The Low and High settings both cook at the same temperature. The only difference is how long it takes to reach the simmer point before the temperature stabilize. So basically, both settings will stop at around 209 degrees and stay there until turned off.

What's the difference between the Low and High cooking: http://www.crock-pot.com/service-and-support/product-support/product-faqs/help-and-how-to-use/general-information/help-and-how-to-use-general-information-faq.html

Follow the instructions for your slow cooker! The person who wrote your recipe doesn't know what your unit is capable of, but the manufacturer does. If they say the wattage is too low to get a roast up to temperature quickly enough, believe them!

As long as you have enough liquid in there, the worst thing that could happen is your roast ends up being too tender. In my opinion, that's a much better option than incubating e.coli. If you're going to actually be around, split the difference. Put it on high for a couple of hours, then switch it to low.

(I'm assuming you mean low for 6-8 hours or high for 3-4 hours)