Again many thanks, but as you probably notice I'm not that experienced. I do have the private key and when I use this in
https://jswallet.github.io/#ThisIsYourSecretBrainwalletTextstringDontShareIt I get a screen of "myriadcoin jswallet" with probably a new wallet address ("myriad coinaddress") with 0 amount. Do I need to send my coins to that address and how can I do that. I'm also wandering how I can open that (new) wallet because there is no seed or private key.
I think I'm failing to understand what exactly your issue is, but here's a guess... You're wondering why
ThisIsYourSecretBrainwalletTextstringDontShareIt sends you to a particular address/private-key?
ThisIsYourSecretBrainwalletTextstringDontShareIt is what is called a pseudo-random brainwallet phrase. The singular private key is derived from this phrase, and the singular public address is derived from that private key. You can send coins to this address, and send coins from this address using the jswallet page. However, Myriadcoin-core is recommended over these types of wallets due to privacy, address reuse, and validation concerns. You can export your private key from the settings of jswallet to make sure you can properly import that address into Myriadcoin-core in case something stops working with jswallet later.
Frankly, if you're just starting out, I would recommend the core wallet of any coin to gain some understanding.