After having installed the app from the Atlassian Marketplace, you can start using Version Sync in right away!

Company-managed (classic) projects: Navigate to Project > Project settings > Version sync

Team-managed (next-gen) projects: Navigate to Project > Project settings > Apps > Version sync

If no versions are visible in your team-managed project, you have to activate “Releases and versioning” first: Project > Project settings > Features > Releases and versioning