Information on how to connect Trezor with MyEtherWallet can be found in this article.
If you have trouble connecting your Trezor with MyEtherWallet, please make sure your configuration meets the following criteria:
If you can't see the desired address with your funds after exporting public keys from Trezor to MyEtherWallet, it means you are accessing a different wallet. Either you have a different recovery seed stored in your Trezor or you are entering a different passphrase.
To access a wallet that includes the address you want to add to MyEtherWallet, you need to use the same combination of recovery seed and passphrase as used when you generated this address for the first time.
You can find more information on issues with passphrase in this article.
If MyEtherWallet is preventing you from spending your funds (signing a transaction), this actually might be caused when the (appropriate) private key is missing. Private keys for signing transactions are derived from the combination of the recovery seed stored on the Trezor device and passphrase (if enabled and used). Hence, if a different recovery seed and/or a different passphrase is used then the private key differs and therefore cannot be used. In such a case, previously generated wallets/accounts in third-party apps would become watch-only wallets without the possibility of spending the funds.
To be able to successfully sign a transaction, you need to use the right combination of recovery seed and passphrase for the address from which the transaction is being made.