There are cables that will connect an unbal/RCA source to a bal/XLR input but there is not guarantee that they will work well in all situations. Cardas makes some as does Hosa.
OTOH, there are adapter boxes for this and these are much more likely to work fine with an RCA cable on one side and an XLR on the other.
The Jensen is excellent http://cgi.ebay.com/Jensen-Transformers-PC-2XR-XLR-RCA-Audio-Converter-/360356008112 but there are cheaper: http://www.google.com/products/catalog?hl=en&safe=off&client=firefox-a&hs=441&rls=org.mozilla:en-US:official&q=rca+to+xlr+converter&revid=1610109775&bav=on.2,or.r_gc.r_pw.&biw=1680&bih=869&um=1&ie=UTF-8&tbm=shop&cid=17143747499655455269&sa=X&ei=FhsJTsLIBMf20gHT0qCOAQ&ved=0CEkQ8gIwAQ
OTOH, there are adapter boxes for this and these are much more likely to work fine with an RCA cable on one side and an XLR on the other.
The Jensen is excellent http://cgi.ebay.com/Jensen-Transformers-PC-2XR-XLR-RCA-Audio-Converter-/360356008112 but there are cheaper: http://www.google.com/products/catalog?hl=en&safe=off&client=firefox-a&hs=441&rls=org.mozilla:en-US:official&q=rca+to+xlr+converter&revid=1610109775&bav=on.2,or.r_gc.r_pw.&biw=1680&bih=869&um=1&ie=UTF-8&tbm=shop&cid=17143747499655455269&sa=X&ei=FhsJTsLIBMf20gHT0qCOAQ&ved=0CEkQ8gIwAQ