Looks like for iOS this is still in beta, so you will need to install "testflight" from the appstore (allows to install beta apps) and then go to https://testflight.apple.com/join/xQfmkJhc on your iphone/ipads browser and accept the invite link. Then you can install mlem, start it and log in (dont forget to type in the address of your instance at the top)
If you are on this post : click on the name of the community at the top (above the post title). Then the community opens and you can click "subscribe". In some communities you need to be approved, then you see "pending".