Official release notes from The Foundry explicitly state that Mac OS X 10.7 is not supported for this version. Users attempting to run Nuke 6.3v4 on Lion typically encounter the following problems:
Older versions of Nuke on newer OS X iterations frequently experienced UI flickering or aliased/garbled node text (BUG ID 11776). Nuke 6.3v4 mac osx compatible with lions
Even if the application launches, a segmentation fault often occurs when attempting to exit the software (BUG ID 20550). Official release notes from The Foundry explicitly state
Support for Mac OS X 10.7 (Lion) was officially introduced in later versions, such as Nuke 8.0v5 and Nuke 9.0v1 . Support for Mac OS X 10
For optimal performance with Nuke 6.3v4, the Foundry Support recommends staying on OS X 10.6 (Snow Leopard) .
Nuke 6.3v4 on Lion is particularly unstable when paired with certain ATI graphics cards. Recommended Solutions
If you must work on Mac OS X Lion, consider these alternatives: