If you want to sum a manually selected (highlighted) cellrange:
1. Click on target cell
2. click on SUM icon on the toolbar,
3. highlight the desired cellrangerange with mouse,
4. hit Enter.
Tibor Kovacs, Hungary; LibreOffice4.4.7 - Portable versions: LO3.3.0-LO5.3.2 and AOO4.1.3; on Win7x64Prof.
Please, edit the topic's initial post, and add the word "[Solved]" at the beginning of the subject line - if your problem has been solved.