To get Mametchi or Kuromametchi, you need to take very good care of your tama when it's young. Play lots of games with him and feed him well. You can also potty-train him so he goes by himself when he's an adult.
Take the best care you can. Get as many potty training bars as possible, play tons of games, no snacks, and always keep it well fed. Oh, and the most important part: make sure to love it! ))
Mametchi is available on all versions, but remember Kuromametchi is only obtainable on v5 to Tama-go.
Mametchi is always gained by perfect care, and Kuromametchi is gained by almost perfect care.
I have gotten both characters several times on all different versions. Just try looking after a boy as well as you can!
If you have a v5, though, get Kuromametchi by feeding Houtugatchi (I think that's how you spell it) sultana pudding. Houtugatchi is gained by good care and looks like an apple.