Expansion ideas unlocks with administrative ideas completed, 34 administrative tech, and not having exploration ideas. Exploration ideas unlocks a lot sooner for the usual colonial powers (21 diplomatic tech), but the diplomatic tech required for colonization increases depending on the colonial region's proximity to your country's colonial region (Indonesia should be around diplomatic tech 31 for European powers).

Unrelated but useful for other people: Exploration ideas in the mod is unlocked by a set of conditions which is displayed at two locations: on the lower part of the ideas tree display and at the top of the Colonial mission tree. The one thing that isn't made obvious is how you need the Commercialization institution embraced for Exploration ideas (makes sense imo, the first colonial endeavors were all entrepreneurial).