From the FAQ
12. I can create dimensions but they show no label.
If you are using Linux, Unix or Mac OS X:
Please make sure that you launch QCad from its installation directory. That's the directory that contains the QCad binary and the directories 'fonts', 'patterns', 'qm', .. Alternatively you can configure the path where QCad searches for fonts in the application preferences as described in the User Manual
Another possibility is that the labels are created but are too small / large. In that case please change the dimension label settings in the current drawing settings as described in the User Manual
If you're using Windows, you might have extracted the ZIP file without using folders. In that case you won't find any sub-folders in the QCad installation directory. Please extract the downloaded ZIP file again and make sure that you activate the use of folders in the options of your unzip tool.