how to tie or slur notes

https://musescore.org/en/handbook/slurs
https://musescore.org/en/handbook/ties

slurs for notes of different pitches, if 2 notes, clik the first , tape S. if severals notes, click the first shift click the last and S, it works if you have severals staves of severals instruments. The play back dont play the slurs.

TIES for similar notes, for ex a quater note C and another quater note C, you click the first and click all above the symbol between the "dots" and the "rests" your note becomes a 2 times C, the play back plays the ties

First, let's check that you are aware of the difference between ties and slurs (apologies if you are aware, but we often get questions here from users who are confused by the two similar looking curvy lines).

A tie is used between notes of the same pitch and effectively adds the durations. So, for example two quarter notes (crotchets) joined by a tie will sound the same as a half note (minim) and a quarter note tied to an eighth note (quaver) will sound the same as a dotted quarter.

A slur is used between notes, usually of different pitches and indicates that there is an immediate transition from the first to the second note, in other words it is an indication that the notes are to be played smoothly.

Right, having got that out of the way we can move to how to input ties and slurs.

To input a tie between two (identically pitched) notes that have already been entered, click on the first note and press +. If you want to enter two tied notes while you are in note entry mode, enter the first note, then select the duration of the second note and press +. This will enter the second note and also add the tie from the first note.

To input a slur between two notes click on the first note and press s. You can then extend the slur to following notes by pressing right arrow while holding down the shift key. You can also select a range of notes (click on the first note and shift click on the last note) and then press s to input a slur covering all the notes.

This is all explained here: https://musescore.org/en/handbook/ties and https://musescore.org/en/handbook/slurs, but since you say you have tried to follow those directions, I hope that my alternative explanation may help. If not, come back here and explain in more detail what is going wrong when you try to enter ties and slurs.