I am on windows (this may be different to Mac, not sure). First you need to download optifine and forge both of the same versions of minecraft.

Then drag both downloads onto your homepage and double click on forge. A thing will open and press 'ok'. This will automatically make forge a version in your .minecraft, versions folder. Then open up the mods folder in .minecraft (if you don't have one then make one) and drag optifine 1.10...... In to it.

Finally to make optifine run when you open minecraft to the starter page click 'edit profile' (bottom left) and change the version you are playing on to forge 1.10.... (This is at the bottom of the pull down thing, which is in the middle of that edit profile page.)

Then hopefully you can use optifine

I actually already updated my modding guide for the staff members, ill just do the same with the public version, so find an update any mone tnow here for a guide with optifine (and shader) plus a great minimap: How to mod MC: Optfine + Shaders + Minimap