How do i change a decimal into a fraction on a casio 300ms

I am afraid there is a misunderstanding. The sequence you describe changes the decimal marker from a dot to a comma. It does not insert a thousands separator.

You cannot.

Display Format

To change the exponential display format, press the [MODE] key 3 times.

Fixed number of decimal places:

Press 1: Scientific Notation: 0

Press 2: SCI: 0

Press 3: NORM

You will then be prompted for NORM 1 or NORM 2. CHOOSE NORM 1

To return the calculation mode and setup to the initial defaults, press [SHIFT] [CLR] [2]

[=]. This will return the calculator to the following settings:

Calculation mode COMP

Angle unit Deg

Exponential display format Norm 1

Fraction display format a b/c

Decimal point character Dot

Percentage Calculations

Divide the numerator by the denominator.

--W/D--

Press the S--D key to switch between decimal and fraction forms.

To convert a fraction to a decimal or a decimal to fraction you use the [a b/c] key.
While converting a fraction to a decimal number is always possible, converting a decimal to a fraction may, at times, not be possible.

Use the [a b/c] button to convert a displayed decimal number to a mixed fraction. If you want to convert further to an improper fraction press [SHIFT][a b/c] to access (d/c).

The sequence [SHIFT][a b/c] (d/c) toggles between improper fractions and mixed fractions.

To convert fractions to decimals, and decimals to fractions use the [a b/c] key. If the total number of symbols (num, denom, fraction bar) to display a fraction exceeds 10, the fraction is automatically converted to decimal format.

Did you set your calculator's format in to a fraction? If you did, change the calculator's format in to decimal.

Hello,
To make sure that results are always in decimal format, put a decimal mark (point) at the end of one integer.
ex: 1./2 or 1/2. or 1./2. display as 0.5, so does 1. + 1/2
To convert from decimal to fraction and from fraction to decimal press a b/c
To convert from proper to improper fractions use [SHIFT][d/c]
Hope it helps.