Hello Damian,
you can always write to our technical support team (support@e-sankey.com) to get quick help on technical issues.
I can just guess from your post what the issue is: If the installation was not completed properly, then the template files in the folder C:\Users\<USER>\Documents\e!Sankey\<VERSION>\templates may be missing. In this case the "New Diagram" button and command can not be used.
To check this:
1) on the Start page, is the top left section "New Diagram" empty (or does it show any diagram templates)?
2) are there any .santem files in template folder (see above)?
If you don't find any .santem files, then the installation should be repeated with administrator rights. Also, make sure that after installation the application is started once still with administrator right. On the first start of the application the template files that come in the installation package are copied to your user directory.
Best regards,
Peter