The page you are looking at in Enterprise Manager is almost certainly reporting free space by comparing
DBA_DATA_FILES (which tells you the total sizes of the various files allocated to a tablespace) with
DBA_SEGMENTS (which tells you the total size of all the segments allocated to objects in the tablespace). This will not change simply because you deleted some data.
When you delete data, you free up space within the blocks and extents allocated to a particular object. So if you delete 100 MB worth of data from the FOO table (and associated FOO indexes), the size of the FOO segment would not decrease. But there would now be space in that segment that would accommodate another 100 MB worth of inserts into FOO. If you are deleting space from FOO in order to free up space for other segments in the tablespace, you would need to reorganize FOO after deleting the data-- this is a somewhat involved process that likely requires downtime as well as a fair bit of testing. Unless you are permanently reducing the size of an object, it is generally not advisable-- if FOO is eventually going to see another 100 MB of inserts, it's not beneficial to shrink the FOO segment only to have it grow again.
You can use the DBMS_SPACE package to see how much space is available in different segments. That will tell you about empty blocks as well as blocks that are in various states of being filled (0-25% full, 25-50% full, 50-75% full, and 75-100% full). When you delete data, you will generally cause an increase in the number of empty and partially empty blocks allocated to a table.