Hi Thomas,
yt doesn't currently support loading data from Cello without doing some preprocessing work first. If the data fit in memory, you should be able to load the data with the load_amr_grids(), which supports loading arbitrary AMR data. Here's the documentation for load_amr_grids:
I know that Matt Turk experimented with loading Enzo-P/Cello about at the last Enzo workshop in 2015. Here's a script from my e-mail archives from back then that purports to load some cello data (no idea if something analogous will work with your data):
I never tried running this script, so no warranties or guarantees :)
In principle it should be possible to write a code frontend for Cello/Enzo-P and support it as a first class level in yt, but someone would need to spend time working on that.
If you or anyone else would be interested in adding an Enzo-P/Cello code frontend, I'm sure it would be a very welcome addition that we would love to have as part of yt. There are several people who have been working on adding new frontends for other codes recently so you should be able to get some freshly informed advice.
-Nathan