Are you trying to get the ball joint out of the lower control arm, or trying to get the steering knuckle and the lower ball joint apart? The latter is a bit easier. With the tool in place and tightened as much as possible, smack the steering knuckle with a hammer in the spot where the ball joint inserts. You could also try heating this area up. Penetrating oil might help, but may not be able to seep into such a tiny space.

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1982 300D, black w/ palomino leather, 203K
2001 E320, champagne, 105K (Mom's)
1999 Dodge Dakota, squeaky & battered, 142K