plumbing fitting threads

Trying to connect Vado Capsule kit to overflow. The issue is when I am trying to install either of the 2 waste adapters it came with to my existing McAlpine overflow they don't fit. Has anyone installed one of these and has the same issue?

Long story, but I seem to have volunteered myself to repair the firebox in an old woodstove. It's a steel fabrication with a water jacket around it. The water jacket is leaking.... somewhere... No chance of buying a new one. The model was obsolete 20 years ago! First job, is to cap off the...