Explain how the complement can be used to find the probability of getting at least one item of a particular type

The outcome A="At least one" means {1,2,3,4,5n}   where n is the maximum possible outcome.

The only other outcome (B) left (i.e. the complement of the above event "at least one") is (B)={0}.

Therefore, by Kolmogorov's second axiom, A and B are complements, and
P(A)+P(B)=1
therefore
P(A)=1-P(B), or in words,
Probability of outcomes of "at least one" is the complement of the outcome of "0" item.