Tried to add 2 numbers, which included decimals, via the =sum facility and the result was 0... what must I do to rectify this?

this was done in Calc !

Likelihood is that your “numbers, which included decimals” are in fact text which SUM will ignore. Use View | Value Highlighting; text cells are formatted in black, formulas in green, and number cells in blue, no matter how their display is formatted.

If you data was imported via a CSV file, you may have failed to check the import option Detect Special Numbers.

This was not imported via a CSV file…started with a blank workbook !

And what does View | Value Highlighting show?

Replace , for . or vice versa.

See this FAQ.