How to do a blast search for similar sequences?
As stated in the comments here, there is no complete genome sequence for avocado as of yet, though people appear to be working on it.

However, there are a large number of ESTs and doing a BLAST of their translations with the tomato PPO sequence (tblastn) gives some hits.

Here is a link to that BLAST.




  However there are no matches, which may be because it is a different species with a slightly different sequence?


This is precisely the problem that BLAST was designed to solve.