I need to use Helvetica (or another font) for a journal but it is not available.

Two kinds of fonts. Prism will recognize any TrueType fonts. TrueType is the standard font types that are supported by both Mac and Windows computers. Prism also can use OpenType-TT fonts.  Prism Windows will not recognize Adobe Type 1 Postcript fonts or OpenType-PS fonts, but Prism Mac will.

If a journal asks for a font not listed on your computer, you have two options:

Option 1: You can add the specified font to your computer.  Prism sees the fonts on your computer, fonts are not specific for Prism.  As long as it is a TrueType font, Prism will recognize it.  These two links discuss adding fonts to Windows and Mac computers.  After you add fonts to your computer, you may need to restart it in order for the new font to be recognized. 

Option 2: Another option is to use a substitute font.  For example, Helvetica and Arial fonts are generally considered interchangeable.  It may be worth investigating whether the font requested by the publication can be substituted with another font.  This is easier and safer than installing new fonts on your computer. The image below shows a comparison of Helvetica and Arial.