If you install a new roster mod, you must start a fresh save. The database is loaded only at game creation. However, some – like injury tweaks or financial adjustments – can be applied mid-save via the in-game editor.
But a common question echoes across forums like the Grey Dog Software forums and Reddit: wmma 5 mods work
~/Library/Application Support/GreyDawSoftware/WMMA5/ (hold Option + Go in Finder to access Library) If you install a new roster mod, you must start a fresh save