If you're looking for the correct protocol names, you can try running something like tshark -G protocols | grep NAME
… where NAME is the name of the protocol you're looking for. (Refer to the tshark man page for more details on the -G option.)
For example:
$ tshark -G protocols | grep RSL
Radio Signalling Link (RSL) RSL gsm_abis_rsl
So in the case of RSL, it looks like you'd need Dissector.get("gsm_abis_rsl"):call(buf, pinfo,tree)
Of course this doesn't always work, because the same search for MAC does find it:
$ tshark -G protocols | grep MAC
DOCSIS Mac Management DOCSIS MAC MGMT docsis_mgmt
MACsec Key Agreement EAPOL-MKA mka
Radio Link Control, Medium Access Control, 3GPP TS44.060 GSM RLC MAC gsm_rlcmac
ISMACryp Protocol ISMACRYP ismacryp
**MAC MAC mac**
MAC-LTE MAC-LTE mac-lte
mac-lte-framed MAC-LTE-FRAMED mac-lte-framed
MAC-NR MAC-NR mac-nr
MikroTik MAC-Telnet Protocol MAC-Telnet mactelnet
MAC Control MACC macc
802.1AE Security tag MACsec macsec
MPLS-MAC Media Access Control (MAC) Address Withdrawal over Static Pseudowire mpls_mac
WiMax MAC Management Message MGMT MSG wmx.mgmt
DCOM IRemoteActivation REMACT remact
Token-Ring Media Access Control TR MAC trmac
WiMax Generic/Type1/Type2 MAC Header Messages WiMax Generic/Type1/Type2 MAC Header (hdr) wmx.hdr
WiMAX MAC-PHY over Ethernet WiMAX MAC-PHY wimaxmacphy
In this case, it seems you need to look at the source code (unless there's some other method I'm not aware of) in order to find the dissector that's actually registered.
$ grep "proto_register_protocol" packet-*.c | grep "\"MAC\""
packet-umts_mac.c: proto_umts_mac = proto_register_protocol("MAC", "MAC", "mac");
$ grep register_dissector packet-umts_mac.c
register_dissector("mac.fdd.rach", dissect_mac_fdd_rach, proto_umts_mac);
register_dissector("mac.fdd.fach", dissect_mac_fdd_fach, proto_umts_mac);
register_dissector("mac.fdd.pch", dissect_mac_fdd_pch, proto_umts_mac);
register_dissector("mac.fdd.dch", dissect_mac_fdd_dch, proto_umts_mac);
register_dissector("mac.fdd.edch", dissect_mac_fdd_edch, proto_umts_mac);
register_dissector("mac.fdd.edch.type2", dissect_mac_fdd_edch_type2, proto_umts_mac);
register_dissector("mac.fdd.hsdsch", dissect_mac_fdd_hsdsch, proto_umts_mac);