How do you convert ... ?

1 mg/kg or 1 ug/g is one part per million parts. For example, 100 ppm of a compound will indicate that there are 100 mg compound per kg on a weight basis, or 100 ug (=0.1 mg) per g of sample. You can then using simple math convert this to percent concentration also. You can also use similar relationship for solutions also. For example, 1 ppm of a compound will equal to 1 ug/ml of the solution (density not considered here in this example, although if the problem asks for it specifically, you may have to adjust your calculations accordingly.