Are Organic Solvents used in Addition Reactions catalysts?
In the example you give, the dichloromethane is acting as a solvent. Solvents are usually chosen to be inert under the reaction conditions so that they do not react and produce unwanted products. 

There are examples of reactions in which no solvent is used, or in which the solvent is a reagent. An example could be forming an ester from a carboxylic acid, using an excess of the alcohol as both solvent and reagent, in the presence of an acid catalyst.  That said, solvents are usually used as they severely effect the outcome of a reaction. Polar solvents (THF, diethyl ether) promote very different reactions to non polar ones (hexane) due to the stabilisation they provide for intermediates during the reaction, dilution can also play a role, deciding whether inter or intra-molecular reaction occurs.