The SNP you mentioned is a common variant and has no effect on the protein according to dbSNP. The dbSNP database is full of technical details, but you can see the distribution of the two possible alleles in different populations if you scroll down:http://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=7042...
23andMe tests for 41 SNPs in the MITF gene, but none of them are listed in the allelic variants section at OMIM http://www.omim.org/entry/156845
The chips just look for somewhat common SNPs, and they're unlikely to cover the specific mutation in your family. It would take actual sequencing to learn what that might be. It could be unique to your close family, or it could come from a distant common ancestor.
However, you can use chip technology to see if the MITF gene might be implicated in your family. The gene is located on chromosome 3, positions 70014031-70016904.All the family members with the condition should match around that region, but none of the family members without the condition should match (except possibly siblings, who could match because of DNA they inherited from the unaffected parent).
AncestryDNA doesn't show where you match, but you can use your raw data with third party utilities for this purpose.
Although your query is about a specific medical condition, it illustrates how the same techniques used to identify cousins can be used to track any trait. I wrote a column about this for JoGG:http://www.jogg.info/52/files/SatiableCuriosity.pdf