Unless you try to convert a new document into an older one, you should see the compatibility options for your Microsoft Word.
Windows has many compatibility options. You can check their Help Site to see if your question is included in the Help center or in the Forums.
Offering more details would help us. Give us the solutions you tried.
You can install EQ 3.0 to edit equations in older word. Choose EQ 3.0 in object. If you want to make a full convert, you'll need to use a third party software.
Solution: The default format for Word 2007 files, DOCX, creates a compressed group of XML (Extensible Markup Language) files, each of which contains information about the document. It is incompatible not only with earlier versions of Word, but also with other programs that have previously been able to open Word files.
Users of Word 2003, 2002, or 2000 can open Word 2007 files if they install the Microsoft Office Compatibility Pack. (Visit www.microsoft.com/downloads and search for Microsoft Office Compatibility Pack For Word.)
Optionally, you can use the Save As option to save files in DOC format, which is compatible with Word 97 to 2003. Users of other programs can use a file converter such as Zamzar (free; www.zamzar.com).
To enable users of previous versions of Microsoft Word to read your Word 2007 documents without a special download, save them in Word 97-2003 format. If you used features unique to Word 2007, such as themes, citations, and equation building, in your file, these elements will not come through as expected during format conversion, and some information may be lost permanently. Word 2007 will restore as many features as possible when you reopen a converted file, but it cannot restore all lost formatting. If you want to keep all functionality, save two versions—one for Word 2007 and one for users of other versions.
Source : http://www.uvm.edu/~bnelson/computer/word2003/whenisaveafileinmicrosoftword2007olderversionsofwordcannotopenthem.html
Mathtype is a commercial program to convert equations in more than 500 formats.
It is well praised. I have seen many references to this program to deal with Word Compatibility Problems.
MathType has a free 30 day free trial you could use to convert your equations for free.