The TLG database files which Diogenes can access, are not part of the Diogenes program. You must subscribe to the TLG (Thesaurus Linguae Graecae) http://www.tlg.uci.edu/
in order to get the texts from TLG. There are some freely available texts which TLG provides gratis. I do not think Diogenes can link to those freely available TLG texts. An individual subscription to TLG costs $100 US dollars per year.
Even though the Diogenes does not include the TLG files, one can still use Diogenes to lookup words in the Liddell-Scott-Jones lexicon and parse words. (The author of Diogenes, Peter Heslin, is personally hoping long-term to be able to provide access to digitized Greek texts - but there are still many legal challenges. It will likely take many years to resolve this problem. TLG has been digitizing Greek texts since the 1970's).
Perseus (Tufts University) provides many texts online. But Diogenes is designed to use TLG texts, PHI texts from the Packard Humanities Institute http://www.packhum.org/phi/
[url][/url] and the Duke Papyri database. I believe the PHI texts have been released to the public (Perhaps someone can provide a link). I do not know if Diogenes can link directly to the PHI texts, or if they must be downloaded by the individual. I also believe that the Duke database is now publicly available.
In order to use the lexicon or parsing portions of Diogenes without referring to the Greek TLG texts, one must start Diogenes, and select the corresponding item from the "Action" drop-down menu box. Select either 'Look up a Word in the Dictonary' or 'Parse the Inflexion of a Greek or Latin Word'. In the Query box, enter either the Greek/Latin word in betacode or unicode, and hit either enter or click on the 'Go' button.
If you have access to a digital Greek text in unicode format, you may use The Hopperizer utility http://www.katabiblon.com/tools/perseus-hopperizer/
to link that text to Diogenes.