Cpk = Z(short-term) which is sigma level / 3

However, if you are starting with DPMO, convert it to a decimal value(divide by 1,000,000), look this decimal value up in a standard normal curve(z table) and find the corresponding z. Minitab can do this as well. Anyway this is long term z. To convert to short term z which is sigma level:

z(short term) which is sigma level = Z(long term) + 1.5

then you can plug into the equation above to get Cpk