Learn French: Utiliser le câble fourni avec la notice. - Use the cable provided with the manual.

This is the infinitif à valeur d'injonction (infinitive used as a command). In French, general instructions found in user manuals, recipes, or public signs use the infinitive because they are impersonal and not addressed to one specific person.

If the manual wanted to say 'Do not use,' how would we make this infinitive command negative?

You would place both parts of the negation directly before the infinitive: Ne pas utiliser le câble. This is a special rule for infinitives; unlike with conjugated verbs, ne and pas do not sandwich the verb.

Is fourni acting as an adjective here?

Yes, it is the past participle of fournir (to provide) acting as an adjective. Like almost all past participles used as adjectives in French, it must be placed directly after the noun it modifies (le câble).

I thought notice meant a warning or a sign. Why does it translate to 'manual' here?

Une notice is a false friend! While it looks like the English word 'notice', in French it refers to a user manual, an instruction leaflet, or a technical specification sheet. An English notice posted on a wall would instead be un écriteau or un panneau, and a warning would be un avertissement.