Candlepower relate to lumen in the same way that the depth of a pond relates to it volume. Or at least very roughly
The _candela_ is a measure of intensity of light, how 'bright' the light is in one particular direction.
The lumen is a measure of total flux of light, meaning how much light is going in a range of directions.
Going back to the pond analogy, the candela is like the meter, and you can use it to measure how deep the pond is below a point on the surface. If you move your meter stick a bit to the side, you may measure a different depth. As you get to the edge of the pond, the depth reaches zero.
The lumen is like the liter. You can measure the total volume of the pond, say by bailing it out with a container. But you can also calculate the volume if you know the depth of each point of the pond, and the conversion for depth and area to volume.
If you measure the intensity (candela) of a light source in all directions, you can convert that set of values into total flux of light (lumen) by doing the appropriate math (integration).
The mean spherical candlepower is the _average_ intensity of a light source in all directions. It is akin to the _average_ depth of the pond. If you know the average depth of the pond (in meters), and the surface area (in square meters), and you multiply, then you will get the volume of the pond, in cubic meters, which you will then have to convert to liters.
The equation converting MSCP to lumen then falls out of the definitions. A candela is the intensity of a light source which produces a flux of 1 lumen per steradian.* Since there ar 4*pi steradians in a full sphere, the conversion from MSCP to lumen is to just multiply by 4*pi.
The lumen is then defined in terms of watts and the measured characteristics of the human eye. You measure the lumen output of a light source with a tool that measures the candela output from a bunch of different directions, and then integrates.
-Jon
*(Sounds circular, I know. Back to the pond analogy: a meter is the depth of a pond which produces a volume of 1000 liters per square meter. It works because the steradian is a different sort of unit from the candela, and the steradian is a basic unit.)