You can download the Magento 1 extension in the official Magento Marketplace.
In Omnisend account, go to My account > Integrations & API > API Keys and create a new key with all the permissions checked.
Click Save and copy the generated API key. Go to your Magento store > Omnisend Plugin and paste the code. Hit Save. Omnisend will start synchronizing with your online store soon. You can refresh the page of the Omnisend plugin to check the latest synchronization statistics.
The synchronization requires a Magento Cron to be properly configured on your store's server. You should contact the developer of your online store and ask whether the Cron is present.
If not, then you can manually initiate an alternative synchronization by clicking the link in the Synchronization message.
Important: once you manually initiate an alternative synchronization, DO NOT CLOSE the window until it is complete. If you accidentally close the window, you'll need to reopen it and the synchronization will be automatically resumed.
Getting started with Omnisend
Here are the resources for you to get started with Omnisend:
If you have questions, feel free to contact us via firstname.lastname@example.org or start a conversation inside your Omnisend account.