Yo! Total music star veteran here, hopefully I'll be able to help you
Anyway, the V6/Music Star has a whole ton of growth charts that you can easily search up -- for most, if not all of them, Memetchi is required to have been taken "great care" of. Luckily, you're on an odd generation, as you noted in your post - so you're super close to getting her!
So, if you want to know how to obtain Memetchi, you're gonna have to utilize something known as "care misses". Basically, if you let your tamagotchi's happy or hungry hearts drop to 0, every fifteen minutes they will call for your attention - signified by a little beep and the tenth icon on your screen lighting up. If you don't answer the call in these fifteen minutes, the icon will blank and you'll have obtained a care miss.
This link here explains more about care misses, probably better than I did. Scroll down to the
Connection Tamagotchis section and it'll be able to inform you!
If you know what these are and how they work, great! All you need to know is that when your tama is a teenager, you'll have to miss 2-3 calls for attention (if I remember correctly) in order for it to evolve into Memetchi.
Hope this helps!