If there are no weights associated with the hops, then BU likely means HBU (Homebrew Bittering Units), which is weight of hops (in ounces) multiplied by AA%. AA% without weight is incomplete; you have to know how much of the actual hops are going in.
HBU is a handy unit because it accounts for the fact that the AA% can change from year to year. The recipe tells you what HBU you're shooting for and you should know the AA% of the hops, so for the Columbus in the recipe:
5.1 HBU = 14 AA% (or whatever yours actually is) * x oz.
x = 5.1 / 14
x = .36 oz.
This also helps when substituting different types of hops in the recipe. If you keep the HBU and the time it is added the same, then you should get the same IBUs at the end. However, the aroma/flavor profile (obviously) and strength (less hops = less aroma/flavor, though different varieties have different levels) will change.
Also, HBU = AAU (Alpha Acid Units, which is what BYO magazine uses).
Edit: In the recipe, the BU may mean IBU if the recipe came from a printout. The contributions to IBUs may have been calculated by individual hop. However, this doesn't tell you anything about the time it's added, its weight, or AA%, though I guess one could use a program to find something that works. 8.8 to 10.5 total IBUs is relatively low (depending on style).