The two fiber interfaces on a WS-C2950C-24 switch are 100Base-FX which is Fast Ethernet (100 Mbps).
The fiber interface on your media converter is 1000Base-SX which is Gigabit Ethernet (1,000 Mbps).
They do not work together, because they use two different signaling methods. The only thing they have in common is that they both use multimode fiber optic cable.
If you need to connect a fiber port on the Catalyst 2950C-24 switch to something else, you need to use different equipment. Either a 100Base-T to 100Base-FX media converter; or a 10/100Base-T to 100Base-FX bridging media converter.
For best performance, set the 100Base-FX fiber ports to full duplex. That will give you your best throughput vs. half duplex; and you will be able to transmit over longer distances: 2km for full duplex, vs. 416m for half duplex.
If the fiber ports are manually set to full duplex, the copper ports should be as well. Don't leave it to chance; that is to say, don't depend on autodetection on the media converters.
