Did you use the same Postgres credentials for the export as you use to run pgadmin? If not, maybe the account you're using for pgadmin doesn't have read access to the table.
The account would be my first bet, but rather than wait for a long back and forth, here are a few other thoughts I have:
Can you view properties for the table in pgadmin? If so, what can you tell us about it, does it look right?
Can you write an ODBC query in SSMS that pulls back a few rows from the table into a new table in SQL, does that find data?
Can you export a subset of the data to another table in Postgres, just maybe 10 rows so you can see the data is all good and gets committed and not rolled back if some whackey value breaks things 9999 rows into the transfer.
Can you look at the disk size of the Postgres database, does it look big enough to hold as much data as you think you exported?
Can you create a table in pgadmin with the schema you want and target that with the SSMS export? Create the table, insert and read a few rows with pgadmin, then try the export and try to read again.
Post back with the results, as well as the version of Postgres you're using.