Sometimes, despite your best efforts, you can not find the articles you are looking for in PubMed. Using MeSH terms in your search strategy simply does not work. How then can you find what you are looking for?
In such cases, search PubMed using keywords (instead of MeSH terms). PubMed will map the term to whatever MeSH terms it finds on your behalf.
If the search you have in mind involves more than one keyword (eg. the effects of cocoa on blood pressure), you will need to omit the AND in your search string (see PowerPoint demos below).
You are looking for a Cochrane Review on whether cocoa lowers blood pressure. See the PowerPoint slides above on how to locate this article in PubMed.