First of all, use the latest builds of Melodyne and ProTools — nothing older than Melodyne 2.5.0.6 or ProTools 6.2.3.
Configuration

Within ProTools, set the "I/O Buffer Size" to at least 256 samples — preferably 512 or 1024 — and choose a high CPU Usage Limit (75%-85%).

Within Melodyne choose a large buffer size, at least as large as the "H/W Buffer Size" you have chosen within ProTools. Set "Buffer Count" to 8 and "Buffer Preload" to 2. If you get drop outs while using the MelodyneBridge, try increasing these parameters. If you do this in real time, streaming will stop for a second and the two applications will be re-synchronized. [fig. 2]

To establish a ReWire link between ProTools and Melodyne, use ProTools as the master and Melodyne as the slave. Launch ProTools, find an empty track and load Melodyne into the first insert effects slot as a plug-in. See Knowledgebase article #28493 for more information on Melodyne and ReWire.