I'm guessing this is in reference to the source code of MacPaint, which Apple and/or Bill Atkinson and/or other parties have donated to the Computer History Museum in 2005.
By searching online, I found a few bits of trivia:
The original MacPaint consisted of 5,804 lines of Pascal source code,
augmented by another 2,738 lines of 68000 assembly language
It appears that the full source code, in digital form is still not published at this time. The museum, merely has physical artefacts such as rims of computer print-out, notebooks and such. According to a wiki note to this page on MacPaint evolution, the museum has not received the intellectual property rights associated with this, and therefore hasn't felt authorized to release the code in full, even for research purposes.
At this time, the interest for this code is probably more driven by historical / epistemological purposes than for the its potential reuse, even partial, in new software.