AFAIK, it is not available as a online form on the BioMart website where you simply submit your query and get the results, but it is available via a WEB-API call:

http://central.biomart.org/martsemantics/cosmic_gene_config/SPARQLXML/get/?query=encodedquery

Here is the link to the relevant e-mail on the BioMart list: https://lists.biomart.org/pipermail/announce/2011-May/000011.html

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3. Querying an access point with SPARQL

• encode your SPARQL SELECT-query from example 2 for use with HTTP GET (e.g. using http://meyerweb.com/eric/tools/dencoder/)
• submit your query as HTTP GET via the command: curl -H "Accept: application/sparql-results+xml" -v \ http://central.biomart.org/martsemantics/cosmic_gene_config/SPARQLXML/get/?query=encodedquery
• the result will be formatted according to http://www.w3.org/TR/rdf-sparql-XMLres/

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