All the properties in EJS's GUI elements is links to variables defined in the model.
If s the the "String" type variable you want to show, you can add s=(int)(vx*100.+0.5)/100.+" " ; in the constraint page. 
You should set the property for text to be %s%. (If you set it to s, it means to display "s" instead of the contain of variable to s). If you click the button next to the box, EJS will display the possible variables can be used. You can select one of them, and ejs will add it correctly.

If you want to show vx=2.3, you need to add another String variable (to stored the string to be displayed).
For example : define another variable called s_vx. and the variable for velocity is vx(double).
Add s_vx=(int)(vx*10+0.5)/10.+""; will give you vx=2.3

If you just add s_vx=vx+""; No error message will be shown , but the display might look like 2.3000000001
I think you can get the point.
If you use it many times, I would suggest you define a function at "Custom Page"

double dv;
public String double2String(double v){
if(v>0)dv=0.5;
else if(dv<0)dv=-0.5;
else dv=0;
return (int)(v*10.+dv)/10.+"";
}
or
public String double2String(double v,double divider){
if(v>0)dv=0.5;
else if(dv<0)dv=-0.5;
else dv=0;
return (int)(v*devider+dv)/devider+"";
}

Them called it with s_vx=double2String(vx,10.); or s=double2String(vx,100.); ...