The setCellFormula() method does not appear to be included in the HSSFCell class, as documented.
Formulas are deliberately disabled in 1.5.x -- you can create a special build and enable them by changing a constant, but you're better off working off a nightly build of the head as the "Experimental formula support" was highly unstable in 1.5.x (hence why its disabled in the binary and by default in the build). The documentation on the site reflects the head and always will. The documentation as it stood is included in the distribution and you should reference it. -Andy