Finding places in Jakarta, especially smaller buildings not on the main arteries, tends to be difficult due to poor signage and chaotic street names. Sometimes, the same name is used for different streets in different parts of the city, and it's often difficult to find the correct street/address without the postal code/region. A sign with a street name facing you indicates the name of the street you are about to enter, not that of the cross street.
Alleys off a main road are often simply numbered, in a sequence that may not be logical, so a street address like "Jl. Mangga Besar VIII/21" means house number 21 on alley number 8 (VIII) off or near the main road of Jl. Mangga Besar.
If you don't want to waste time, ask for the descriptions/name of nearby buildings, billboards, color of the building/fence and the postal code of the address. If you still cannot find the address, start asking people in the street, especially ojek (motorcyle taxi drivers).