Im a noob myself but ill have a go.
In the same state with get mouse x, add an action 'Set Velocity' (or 'Set Velocity 2d' if it is a 2d game or if youre using a 2d rigid body on your object)
In the set velocity action under x axis put in your variable (the variable that stored the horizontal axis).
From my understanding that should be it, but I only know this from tutorials ive done recently.
Also in case for whatever reason you dont want to use velocity there is a way i have found that is not very elegeant and probably very nooblike but:
I have done it before with an empty game object on the left and right of the screen. You need to get each of their positions into a global variable, then on the get mouse x state have a compare float, if float is under 0 go to left state, if over 0 go to right state. In both left and right state have a move towards action (left one moves towards empty on left, vice versa for right one). Ignore vertical on the move towards actions. Insert a Compare float action under the move towards in both left and right state, have the float compare sending it back to centre and right, depending on </=/> to 0.
As I said the second method is not very elegant, but does not use velocity.
Good luck, also watch some tutorials, this is the first thing most of them cover lol.
Edit:
Sorry I thought you were actually using get axis not get mouse x, ive edites the post so it makes sense now. I hope it works for you, but im now realising the mouse controls may be trickiet than simply using axis. Still check out the breakout tutorial usimg playmaker on youtube lol: