Snake's down special has him placing C4 on the ground that he can detonate by doing his down special again. I have seen it be placed on enemies before, and I have done it myself, but I can not do it consistently. Sometimes I will place it accidentally on an enemy while other times I will try to and not succeed.
The wiki says that you just need to be near to them to do it, but I feel that there is more to it, at least to doing it consistently.
Best Answer
I've got bad news for you: you have to be really close to someone to be able to place C4 on them. How close, you ask? It looks like it actually depends on the opponent's hurtbox--meaning the smaller the character, the closer you'll have to be.
On the training stage with the opponent centered on 0, the furthest I could be was 7 "units"1 for Pichu (one of the smallest in the game):
but 12.5 "units" from Ridley (one of the largest):
These distances also differ when Pichu/Ridley were facing the other way, making me think that this is indeed based on hurtbox size.
So your problem is likely that you're simply not close enough.
You could try to get in and find an opening where your opponent gives you enough time (9 frames) to plant the C4 on them, but then you're almost better off doing something like forward-tilt/up-tilt/grab which all come out faster or even forward-smash which is only 2 frames slower.
Instead, you can use the fact that C4 will still be planted to the opponent even if they're shielding!
So I'd say the most opportune time to use this would be after you've conditioned them to shield when you approach. This way, you can use whatever approach you've been, but mix up the attack for a run up sticky.
Minor trivia I learned while experimenting
Dropping a C4 from above will always make it stick to the floor. Even if you drop it directly over the opponent (much to my dismay--this was going to be my suggestion if it worked).
If a character is "stuck" with the C4 and they pass another character, the C4 will transfer to the new character, even if this character is Snake himself! So be careful after you stick someone or when you detonate, you may be killing yourself! However, after a short window (I estimated about 20 frames), the C4 can be passed again. So even if it gets passed back onto you, you could try to stick it to the opponent again by getting close to them.
The C4 will fall off of the character after a certain amount of time, so if it gets transferred back to you, don't fret. You'll just need to wait for it to fall off.
Shielding can change the size of the opponent's hurtbox, so the "stick range" may change slightly if they're shielding--either way, I'd suggest playing safe and getting as close as you can.
Pichu puts her ears back when shielding to fit into the shield and it's the most adorable thing.
1. I'm calling each of the smallest squares a "unit" here.